There's a lot of things I like in PF2, but I'm not mentioning them here... why bother? If you don't see something mentioned, assume I don't want it changed. :) (As a programmer, I'd be really irked if QA sent me message after message of "This works just fine!". The job of testers is to find bugs. And that's what I'm doing.)
* ATTACKS OF OPPORTUNITY. No, no, no. This is a real deal breaker. Making this just a fighter shtick (or given at random to monsters) undercuts one of the core mechanics of D&D since 2000. Yes, the rules can be complex, especially when reach is involved. There's room to simplify them without eliminating them. This dramatically reduces much of the tactical thought involved in PF combat. Points where you must decide to take a risk - "Do I stand and get AOO, or lie here on the ground and hope someone distracts the guy next to me?" "Do I risk casting defensively or take a 5' step back, which puts me in a less useful spot?" - enrich combat. Removing AOO for all but a few specific monsters, and then, only for moving and nothing else, is a HUGE, negative, change.
* Hyper-siloing. The reason I went to PF over 4e was because I liked the freedom to build characters from a relatively granular set of building blocks. I could easily take a feat or invest in a skill outside my alleged "role" and, as a GM, i could build complex and rich NPCs, even for low level commoners, due to the choices I had. PF2 strips this out. And it's going to scale poorly. Very poorly. Two years from now, as you introduce new classes with overlapping roles, you're going to either be flooding the game with effectively-identical feats for combat, magic, and roguery, or you'll have things like "Swashbucklers can take these rogue feats or these fighter feats, but not these other ones"... and then update it for every new feat you add. (Heck, Fighters and Rangers already both get Double Slice. I haven't seen if there's more examples. And this means you end up printing the same feat twice. Why not treat feats like spells? So:
Double Slice (Fighter 1, Ranger 1, Rogue 3(hypothetical))
Power Attack (Fighter 1, Barbarian 1(hypothetical))
All in one section?
(Yeah, barbarians don't get Power Attack as a feat currently... which just reinforces my complaint about siloing, because "BRONK SMASH!!!!" is so iconic it makes my teeth ache. No, wait, that's the pack of jelly beans I just ate. But it's iconic as hell. Ditto my Rogue/Double Slice example.)
And one of the things I really liked about PF, especially in the later classes, was how many classes got their own mechanics. barbarians have powers, rogues have talents, gunslingers have deeds... now, everything's a feat. Again, to draw on my programming experience: Constantly adding new properties and methods to a single class in order to avoid having "too many classes" (which can be programming or RPG design...) is an Anti-Pattern. (As is making up a new class (again, code or RPG) for every minor variant.)
Lesser peeves, in no particular order:
* Size adjustment to AC and damage (and combat maneuvers, etc.). The lack of this was a real issue for me in SF, and I'm sad it's been carried over. The rules were simple, they were generally calculated once (size change is relatively rare), and helped create some verisimilitude, making greatsword-wielding kobolds rare compared to stabby sneak attacky kobolds. I suspect the change is due to the desire not to impinge on "freedom" and not "force' someone who wants to be a halfling or goblin to avoid "tank" type classes/roles. There's better ways to do this, such as a few special feats intended just for those narrow situations.
* Bulk. Yeah, calculating encumbrance is a b!&!$ and no one does it once anyone in the party has a Handy Haversack. I still find it too abstract. I can live with this,but I wanted to at least whine a little.
* Skill Point Removal: I do not get the Skill Point hate. I *like* the granularity of it. I love skill feats, I love the idea of ranking skill by expert, master, etc., but you can do this and keep granularity! (Yes, I say "granularity" a lot. I get paid by the American Grain Association. They think I'm writing about corn futures.)
* Monster Building: I really prefer that monsters and NPC use the same system as PCs, though an optional "build by role" system is fine. Again, this was a real issue for me in SF, because the system wiped out most of the racial and class features. An ogre soldier NPC and a goblin soldier NPC were pretty much identical. Both could use the same weapon with the same skill and had the same stats and melee attack bonuses. Maybe there were 1 or 2 small points of difference, but nothing compared to what you'd get trying to build a 5th level goblin fighter and an equivalent ogre fighter (allowing for racial HD) in Pathfinder. The flat skill bonus for all non-listed skills lead to things like any relatively high level monster having huge bonuses to every skill. (And when you remove the size modifiers for many things, as noted above, a purple worm has a quite respectable +14 to Acrobatics, as opposed to the -2 it has currently. ("But the GM is supposed to just decide not to allow that!" Yeah, yeah, yeah. Arguing "The GM can ignore the rules" is like saying "Look, if your word processor crashes when you use too many 'q's, stop using so many 'q's!" We're in beta. Lets point out bugs and fix them if we can. If the product ships with bugs, THEN, we can discuss workarounds.)
* And that leads to the next peeve, I guess: Too much homogenization. In order to avoid (I am guessing) the problem of "This spell will kill half the party and be ignored by the other half"/"Half the party can't hit this monster, the other half can't miss it"/etc, you compressed all the numbers so that high and low saves, attack rolls, and so forth are all within a few points of each other. The "your level is your base bonus" mechanic means everyone is generally good at everything, and the skill rank bonuses (trained, expert, etc) are VERY close together, so that a 10th level Trained Acrobat with a decent Dex is only two points away from a 10th level Legendary Acrobat. ("But that's what skill feats are for!", you say. I like skill feats, a lot more than class feats. But they grant generally exceptional abilities and tricks. Meanwhile everyone's about as good as everyone else at everything. A range of level -2 to level +3 is not a good range.)
And I get the problem you want to solve: "Well, Fred the Fighter never put any points into Climb, so, this DC 10 cliff is suddenly an obstacle for the party when it shouldn't be at this level." or "One guy in the party with a +20 Perception is never surprised, and the rest of the party always is." or "No one put any skill points into social skills (or the one person who did is elsewhere) so every encounter is a fight."
But for those of us playing since the dawn of time (in my case, 1978), things like this are features, not bugs. (Or maybe they are bugs, but we've learned to live with them, and fixing them makes things feel strange and wrong.)
This problem of "you're either dead or immune" is already addressed in PF2 by making most effects have progressive or weaker effects unless you roll really, really, badly. The Medusa, for example, is still terrifying, but it can't wipe out a party with one round of bad rolls.
*BAB, Or Lack Thereof*: Yes, when you figure in Str bonus and (to a limited extent, see above) proficiency, a Fighter and a Wizard aren't going to be as good with a broadsword, but "your level is your BAB" removes much of the "flavor" of D&D-derived games.
Oh, BTW, Paizo - how come your books are among the most beautiful and professional on the market, but your forum would embarrass someone running a BBS on their TRS-80? I want to be an active, and hopefully helpful/positive force for the development of PF2, but this forum is like pulling teeth. I couldn't even get into it on Firefox, I had to switch to Chrome. No WYSIWYG editor? Seriously?