I'm approaching how I participate in this playtest a bit differently than I have in the past (after not really participating in the last one). I find the playtest forums a little stressful at times, so I'm going to try and just get all my thoughts in one place for the developers to see rather than try and sway people to my viewpoint. Obviously as this is being posted so soon, this is without playtesting, and I probably won't get to playtest it as I don't get to be a player often and none of the PCs in my campaigns would really switch over to this. Anyways, this is going to be pretty in-depth and long, so it will be sectioned off into spoiler tags.
Core class and general thoughts:
While it does present some mechanical issues I'll get into when talking about Elemental Blast, I think Constitution should be the Kineticist's core ability score. It seperates them from other magical characters, and shows that the source of their power is wholly different.
Having the initial proficiencies of a martial (minus martial weapons) feels perfect. Glad to see that they still get light armor, though the lack of medium or heavy armor could make going into melee harder for strength focused characters.
I enjoy the decision making of getting more elements for fewer starting feats. And Universal Gate getting to change their choice daily feels fitting and also somewhat worth the tradeoff of only getting one feat.
Gather Element feels like a sufficient cost to getting to use your abilities without being overbearing, similar to a stance, and interacts nicely with the Impulse and Overflow traits. However, it would be nice to be able to do other things while doing it, like Gunslinger reloads or Magus recharging.
One thing about the Impulse trait is that since it gives the manipulate trait, it makes using Elemental Blasts a lot more punishing for melee characters not using Elemental Weapon. Also, I don't think I like auras only being usable in encounter mode. It would break immersion for my players not being able to use all their tools unless they're in a fight.
In general, I like how Elemental Blast was designed. I like that it's clearly laid out how to use it melee or ranged, while not feeling like you're using two different things regardless of how you use it. Furthermore, I like how it scales with unarmed proficiency and how it scales using handwraps. There are some quibbles however. Firstly, the damage types. I know wood, metal, and hybrid blasts will be coming in the book, but there are some types, such as cold, that don't feel like they would fall under that. I'm hoping to see some feats in the final product that allow you to switch the damage type of your blast. Secondly, you don't use Constitution for anything here. Not using your key ability for your attacks has proven to be fine for the Inventor and Thaumaturge, however they both get very large boosts to their damage from their class features. The Kineticist doesn't appear to have anything like that, and could leave their Blasts falling by the wayside in favor of other at-will abilities that do use their Con. This issue is compounded by the fact that, at levels 5 and 6, their proficiency lags behind other martials for no clear reason, especially considering they get master at 13 just like everyone else. Final thought on the blasts, is that Air seems to be paying for reach when it really doesn't need it. I suppose it helps with flanking, but given that you can use ranged blasts at any time I feel like it would feel better if it was a d6 damage die and dropped reach like fire. (EDIT: On reread, the 120 foot range is also probably contributing to the lower damage, which makes sense. However, that doesn't change that melee air will feel pretty bad.)
The rules on how impulses interact with effects that deal with spells feels fair and clear.
Impulse feats scaling with level like spells do is great. In fact, I would love to see this in place for future feats for other magical martials like monks and thaumaturges.
Adapt Elements feels like it gives enough flavor options for your elements to be fun. I can already picture my players trying to see if they could bring a live shark into town using the Sustain option (though I might suggest renaming that, as at a glance I assumed it was to sustain the other effects).
Not having to spend a feat on getting resistance, and later immunity, is nice even if it is only with your element gathered.
Will require playtesting or math to figure out how actually effective it is (I'll leave that to other people) but the inclusion of Extract Element is great and will make single element kineticists a lot more playable.
The critical specializations are nice but feel a little imbalanced. Earth feels stronger than the rest and air feels weaker. The fact that it's impossible to ever get more than one critical specialization seems odd. Most players who pick multiple elements wouldn't want for one to be better than the rest.
The proliferate option for Adapt Element seems nice, but hazardous terrain isn't a term used often in player-facing material so it might be helpful to clarify exactly how it works.
Kineticists getting Fighter-like flexibility feels good. I wouldn't have thought of this but like it a lot. Might step on the toes of Universal Kineticists though, so I might suggest making them the best at this.
Pure Adaptation feels a little limited in scope for an 11th level ability. Plus, the requirements and effects seem a little unclear, particularly with air and fire. What does 1 bulk of air or fire look like? Why exactly would fire need to be purified? If you purify air, wouldn't nearby, non-purified air just fill in the space?
I love that the effect of Final Gate is on a class feature rather than a feat.
Finally, to talk about what isn't here. Burn. Personally, Burn is a super evocative part of the class for me, and a lot of the flavor presented here still seems to imply that you don't have full control over your element. I understand that a lot of people are opposed to Burn, and it might not be the narrative direction you want to go with the class anymore, but if it doesn't make it into the final product in some way I hope we can get a class archetype that adds it.
Kineticist Feats:
Elemental Familiar: Exactly what I would want from a feat that does this.
Elemental Weapon: I like this but have a small issue, that being the one handed restriction. I know you all want this to be a side-grade to Elemental Blast, but I just really want a big hammer made of stone. Maybe you could allow using two-handed weapons but you lose access to Elemental Blast while they're out? Dual wielding would also be nice.
Flexible Blasts: I really love that this is a thing, but it does feel required for earth and water kineticists who want to use both melee and ranged.
Kinetic Activation: Nice little feat, not every character will want it but I definitely know some scroll addicts who will want this.
Voice of Elements: Feels very campaign specific, as not every game will have elementals all about. Could we maybe get a speak with plants-type effect on this, but for your element?
Blast Barrage: Solid action economy saver.
Command Elemental: Again, campaign specific, but I see little way to make this less specific so it's probably fine as is.
Cycling Blasts: Playing with multiple elements and wanting to use all of them seemed clunky to me at first, so I'm glad this fixes that. I don't think the power justifies 6th level though, considering it's basically Quick Draw.
Stoke Element: Seems great as a third action if you're in a situation where you're basically just a turret. Little disappointed this is exclusive to Dedicated Kineticists, and after looking over the impulse feats, I think damage will be too low for characters that don't have this.
Aura Shaping: We don't have a lot of ways to exclude allies from AoE effects in PF2, so I'm happy to see this here. The extra area is also nice, but wouldn't be the main reason I take the feat I don't think. I won't really be able to say how good it is though without seeing how good the aura feats are.
Chain Blasts: I was low on this feat until I saw your MAP doesn't increase. I love this feat and will take it on every Kineticist I build. If electric blasts don't make it into the final product though this will make that all the more obvious.
Deconstruct Element: Seems good, but effectiveness will vary massively between elements. Even after Rage of Elements I think the amount of wood effects I throw at my players will be very rare, for instance.
Fusion Blast: My players will take this, feel bad about it missing, but feel amazing about it critting. I doubt your average forum-goer will like this but among more casual players I think this will be a popular feat.
Gather Amalgamation: This is super cool, but the two action cost feels kind of high. It seems it would be best to be using as many overflow impulses as possible on this, but unless you know at least three different overflows you aren't really saving any actions.
Rapid Reattunement: Pretty cool, and I didn't expect the Kineticist to actually be better than the Fighter on this front.
Flowing Kinetics: I've been wanting a feat like this for casters for ages, glad it's somewhere at least.
Steadfast Kinetics: Feels on par with the feat the other elements get.
Effortless Impulse: Nothing to say about this, we've seen this feat many times before.
Imperious Aura: Again, not sure how good auras in general are, but this is a great feat to support them.
Maelstrom Blast: Exactly what I want a high level kineticist to do. I do have one question though, if you have Gather Amalgamation active, are you able to make each blast a different element? Sounds cool but a bookkeeping nightmare, so I could see you guys going either way on this one.
Nourishing Gate: I like the mechanics of this one, but I'm unsure of the flavor of it. It feels like this was supposed to just make you an elemental at one point in development, and I might like that idea more.
Flawless Element: This is one of the best action economy savers in the game, but it still doesn't feel too powerful due to the one overflow rule. A capstone I would look forward to.
Omnikinesis I don't think this has as much raw power as Flawless Element, but this will absolutely make you feel like the master of your elements. Universal Kineticists will feel incredibly versatile once they get here.
Air Impulses:
Aerial Boomerang: Cool and decently powerful for a 1st level feat, and I love how this rewards creative positioning. A bit unclear on what happens if you use this in a room smaller than 60 feet though. Does the boomerang hover in the furthest possible spot, fail to return, or what?
Air Cushion: Having to already have your element gathered will make this awkward at times, but overall pretty good for an at-will feather fall.
Fair Winds: A very cool feat and one that will be powerful in a party full of melee characters. Not entirely sure why the bonus is higher for allies, but I think I would rather have that than the reverse.
Whisper on the Wind: Super cool, but niche enough that I can't see myself taking this unless I'm a dedicated gate or have Natural Ambition.
Flinging Updraft: Another nice support ability for your melee buddies, but I can't help but feel a little sad that they don't take falling damage.
Soothing Breeze: The healing starts out a little bit lower than 3-action heal, but pretty quickly gets outscaled. At level 4 this is healing 1.5 less on average, at 20 it's 17.5 less. I understand that this is resourcelss, but it's still not spammable so some higher scaling to make it useful at high levels would be lovely.
Clear as Air: Very solid, not much else to say.
Storm Spiral: The fact that the only damage is electricity makes the forced movement feel a little out of left field. I'd like a rider effect more akin to something that feels like it came from lightning.
Celestial Palisade: The only issue I have with this feat is the name. While technically accurate, celestial makes me think of outer space and the stars, not so much the wind.
Wings of Air: A nice feat, glad they get at-will flight earlier than usual.
Circulate Qi: An okay feat, but reducing conditions by 2 on a crit success would be welcome.
Wiles on the Wind: Feels weak for a 12th level feat. Fascinated isn't a very strong condition.
Body of Air: Great out of combat utility, okay combat utility. A fine ability to have at will.
Ferocious Cyclone: Just a solid damage ability.
Infinite Expanse of Bluest Heaven: First off, all these 18th level feats have great names. But I'm not sure about this effect. First, enemies having a 50% chance of the effect ending on their first action after failing their save doesn't feel very good. Secondly, even if they fail all their checks, the mental image of them just running around seems silly. If they feel like they're falling, why are they able to run?
Crowned in Tempest's Fury: Feels like an alright capstone on its own, but seems amazing when combined with Aura Shaping. Expanding this to a 30-foot aura that ignores allies makes this a very worthwhile feat.
Earth Impulses:
Geologic Attunement: Way more usable than I expected. The fact that moving doesn't permanently end it is very nice.
Stepping Stones: Ways around difficult terrain feels very earth kineticist, and this works decently.
Stone Shield: I notice that it doesn't have the 10 minute time limit after blocking like Shield does. I assume losing your gathered element was considered punishment enough, but thought I'd mention it anyways in case it's an oversight.
Tremor: Comparing this to Aerial Boomerang, the area is much smaller and the damage ceiling is lower, but you get difficult terrain and potential prone out of it. I don't think it's bad, but it certainly isn't something I'd want to use all the time.
Dust Storm: Seems good, but I need clarification on how this works with Aura Shaping. If I exclude four of my allies from the negative effects, are they still concealed themselves but able to clearly see everyone else in the area?
Restoring Mud: This is... not good. Including the temp HP, this provides 9 HP on average, while 2 action Heal and Soothe give 25 and 19 respectively. Soothing Breeze, for 1 more action, only heals 1.5 less but to everyone within 20 feet. And Restorative Mud will get even worse over time. It doesn't need to heal as much as Soothe, but it needs to heal a lot more than this.
Igneogenesis: A fun flavor feat, but the fact that it can never be permanent is disappointing. Maybe allow it to be permanent if you spend 10 minutes doing it or something? This feat just inspires sculptor characters.
Rolling Boulder: Compared to Aerial Boomerang cast at the same level, this is lower damage, a smaller area, and a feat 5 levels higher. Is that worth forced movement and moving the distance of a Stride? I'm not sure it is.
Spike Skin: I've always liked "punish for damaging me" type effects, so I like this as well. The 1 hour immunity feels really long though, and annoying to keep track of.
Swim Through Earth: I've always wanted this kind of effect on a PC. The slowed condition doesn't have a value, though.
Hurtling Rockfall: Again, earth's damage options are not up to par. This does half the damage to one creature what fireball does to a 20-foot radius sphere. And there doesn't feel that much reason for the shrapnel to scale so much slower than the sphere. If one has to scale slower, I'd prefer the sphere.
Rock Rampart: Pretty solid, I like how long the wall is compared to similar options.
Assume Earth's Mantle: This is very cool. Might feel bad sometimes using an entire turn to set up, but the benefits are likely worth it.
Stone Guardian: A fun feat, though it doesn't specify what kind of cover this gives. I assume standard, but if the answer is lesser this becomes less good.
Rebirth in Living Stone: Very fun, but can this just last a minute? This is nearly the apex of your power, and the free action Gather seems nice until you realize you're spending that action to Sustain anyways.
The Shattered Mountain Weeps: Finally, earth gets decent AoE damage! One issue though. The flavor of this is rocks falling, but this being a burst rather than a cynlinder makes the space that deals damage very weird.
Fire Impulses:
Burning Jet: Situationally useful before the 60-foot version (where it's always better than Striding twice) but cool in any case. The big issue is that, while the movement doesn't trigger reactions, the Impulse trait on this does.
Eternal Torch: This feat is cool flavor-wise, and I love how it scales. Could light up your home with these eventually. What I'd expect from a 1st level feat.
Flame Eruption: You can put this in an intersection between squares, right? If not, this is awful, if so, this is alright. Scaling every three levels feels too low, though.
Warming Nimbus: A buff to you and your allies to the most common energy types is always welcome. Funny how this effects enemies, and you could use Extract Element to take away the resistance you gave them.
Blazing Wave: Pretty good, I think. Lower damage than Burning Hands but much better area.
Desert Shimmer: I like this, concealment is a pretty powerful condition. Similar question do Dust Storm regarding Aura Shaping though.
Crawling Fire: There's some issues with this. For one, the Crawling Fire can apparently be dealt damage (which gets transferred to you) but does not have an AC or saving throws. It allows you to use impulses from farther away, but those three actions could have just been used to move up and back. Not to mention fire blasts and most fire impulses have respectable range anyways. It's not like you're putting yourself in more danger, since it can take damage. If this was just a firey Spiritual Weapon that you could cast other impulses out of, it would probably feel better.
Wandering Smoke: I love that you can use this on allies, and on yourself it's a pretty solid evasive maneuver. You probably don't want this and Burning Jet though, and it also has the same reaction issue as Burning Jet.
Kindle Inner Flames: I'm unsure how I feel about this one. The buff is somewhat better and easier to apply than Fair Winds, but that's a 1st level feat. But at the same time, this doesn't feel super bad to me. I don't really know how to evaluate this.
Solar Detonation: The damage and scaling isn't great, but that would be made up for due to the conditions... If it wasn't incapacitation. I don't hate the trait as much as some do, but it feels bad on a mixed damage and debuff ability like this. Maybe swap the blinded for 1 minute with something like "blinded for 1 round and dazzled for 3" and remove incap?
Architect of Flame: The first wall impulse I haven't been a fan of. Compared to an equivalent level wall of fire, you're sitting at half the damage and a third the area (height doesn't usually matter). I know it has to be worse, but this feels like too much.
Furnace Form: A good self buff. I like that it's only two actions and not overflow, even if it is only until the end of your next turn.
Arrive in Conflagration: Fire loves it's GTFO spells, doesn't it? Ignoring the issue with impulse and reactions, it feels weird that Fire is slipperier than Air.
Horrid Ignition: Your blast is likely doing 3d6 damage as is at this level, so getting that in persistent damage is worth the extra action, particularly with the debuff.
All Shall End in Flames: My first instict was to say the damage is too low (I think I'm just used to saying it) but this actually seems to compare pretty favorably to a ranger using Impossible Volley.
Ignite the Sun: Damage-wise, this is, well... It deals the same damage as a 3rd level Flaming Sphere. Sure, you might be able to hit more things with this, but it only moves 30 feet on Sustain. It's 18th level, you're fighting creatures that can move all over the place. Not to mention the buff area is only a 20-foot emanation. Good luck hitting both your barbarian on the dragon's back and your archer fighter with this. I want to feel like I'm creating a grossly incandescent sun of my own, not a somewhat better Flaming Sphere.
Water Impulses:
Deflecting Wave: Resistance on a reaction is pretty unique, and it feels like this grants enough of it.
Tidal Hands: This is actually one of my favorite damage abilities here. The damage feels on par and I like the variable area.
Water Dance: Solid on an ally, but when using it on yourself it has the impulse/reaction issue.
Winter's Clutch: I overall like this, but choosing specific squares to make difficult terrain feels weird, especially since it never scales past three.
Return to Sea: Feels pretty good if niche, a good target in some campaigns or with elemental flexibility.
Veil of Mists: I like it, but would like it more if at some point in the scaling the immunity goes away.
Slippery Sleet: The damage isn't great on this, but the damage doesn't matter. What matters is that the Balance DC is 15 and doesn't scale! If that was your class DC I think you could take away the damage and I would still take this.
Standing Surf: It's a wall, and pretty average among them. Would probably be bad if it wasn't at 6th level.
Lowland Fog: Does this not work, or am I just not understanding it? So, it makes everyone in it hidden. But then as soon as someone attacks, now no one is hidden? It mentions guessing the location of the target, but that's not how the hidden condition works. At 14th level letting your entire team Stride for free is pretty nice, but I'm aside from that I don't think this works as intended.
Torrent of the Blood: Solid if you're facing a lot of poison or disease, but mediocre otherwise. The healing is very low. Also, you can't be in the area of it.
Drowning Sphere: Aqueous Sphere is good, and while this is a lot higher level than that, the fact that this doesn't overflow feels nice, so you can actually cast this at will. You also feel more equipped to take advantage of the flat-footed than a caster. It might be a little high level for the effect, but I don't think it's bad.
Glacial Prison: This feels worthy of the incapacitation trait, though given that the main effect ends upon taking a hostile action, I think the crit fail effect could be nastier.
Barrier of Boreal Frost: This is interesting. This wall impulse scales exactly how the base spell does. This feels good, but is there any reason the others don't work the same way?
Sea Glass Guardian: Comparing to Stone Spikes, this trades the resistance for better duration and applying to everyone within your aura. I'd say that's pretty good!
Ride the Tsunami: I don't think this is bad, per se, but I don't think it has that "wow" factor many of the other capstone feats do. Like, compared to All Shall End in Flames, it's similar damage but is ultimately just a bigger Tidal Hands, while All Shall End in Flames incinerates everything it kills and can give you a phoenix-like effect.
Usurp the Lunar Reins: So... this 18th level feat uses 3 actions to create difficult terrain in a 20-foot burst of water. For such a fantastic feat name, this is a super underwhelming effect.
And that's all! Overall, I like the class, particularly in structure. I do think it needs fine tuning in a lot of places though, especially damage, and impulse triggering reactions thing really needs to be addressed considering how many abilities are about avoiding reactions. Apologies if I missed or misunderstood anything, this was a hefty document to take in all at once. Anyway, I wish I could write a more detailed closing thoughts, but I've spent 3 hours writing this and my hand is cramping. So long, and thank you devs for the class!