I'll use the same format for my thoughts.
D20 System/Everything is modular, and based on feats: I like these as well, nothing to really add other than excitement.
Different Modes of Play: It will be interesting to see how this actually plays out. I feel like most tables loosely ran like this it just gives actual rules to it. It will probably require fine tuning so players can actual make money and/or items of value. Could remove the need to find piles of gold everywhere you go.
Action System: I personally am excited to see what they do with this. I expect feats to play greatly into what you can do with the actions allowed. Sudden charge combining 2 strides and an attack into 2 actions is a nice starting place. I'm also curious to see how this plays with haste, as in, can you sudden charge twice now or does the bonus action have to be exactly a stride or attack. This system gives the building blocks to make some fancy maneuvers.
Class Design: I think the classes are pretty solid where they are. I honestly think they may make it difficult to add many more of meaning. I'm the opposite on Paladin, for the most part. I disagree with their choice of LG only. I really like the Righteous Ally ability line though, I believe this can make some pretty interesting characters and adds an additional RP element to play with. The Litanies can make a pseudo support caster paladin, possibly able to stand back and guard their allies to make use of Retributive Strike (which I'm not crazy about but we'll see how it actually looks). I think it's important to look at all the abilities a class has and realize that feats will probably be able to enhance any or all of these. So you could ignore a feature you don't like to further enhance the ones you do.
Martial Combat: I personally prefer Martial characters over Casters, even if its a Cleric I know I'll be wading into melee. The 3 action system section talked about how I felt about feats affecting action economy and I'll re-emphasize that here. I think early on, I agree, you don't have a large amount of options to actually do. But as you progress your feats will open up tactical options. Sudden charge spares you a feat to get you into melee, Pirate Archetype gave a similar one with a few more requirements. I'm hoping each Martial class will have a few things only they can do to adjust their action economy that is accessible via multi-classing. I think that is the only way for martial to martial multi-classing will be competitive with Martial to Caster multi-classing. For instance, maybe Rangers get an upgraded Double Slice to use as their 3rd action to do 4 total attacks, 2 with each of their weapons, in melee. Or combine Sudden Charge and Double Slice for a 3 action activity to get them into the action with reduced penalties.
Spellcaster Combat: I initially thought Spellcaster combat seemed bland in purely moving, or raising a shield, and casting with the only choice being which spell. But the ability to adjust how many actions you can cast a spell with does open up more options beyond that. Hopefully some buddies pick pure casters so I can see it in action but they are generally in short supply in my group.
Equipment: I'm mildly excited about the new item system. I don't think I've seen enough info to make any judgments though. Just hope Trinkets aren't overly expensive because they don't see too exciting yet.
Archetypes: I'm very interested in seeing how Archetypes play out. I think they need a separate Dedication than Multi-classing, but the Playtest will decide that. I feel like Archetypes will be very hard to balance and be desirable by more than a few classes. I hope they can make it more than an RP decision. I also hope some will be acquired by Skill feats and others by Class feats. Could allow it little more variance and strength and make some otherwise obscure ones more desirable.
Overall, I am very glad they are updating to second edition and am pleased with their method to do so. I think most games need a new edition every so often to condense the amount of material needed and put their experience to use to make a better system.