The Skill Tree. It's very barebones, you get some core skills and a couple passives here and there. If they have interactive bonuses, i'm not aware of them yet so for now I will assume they don't. However, I do like how they make you choose only one bonus for each skill, it adds a little variance and agency but overall it doesn't feel very inspired. I made a Cold Sorceress and was pretty unimpressed. It's not offensively bad, but it's just not the greatest. It reminds me more of FF10's sphere grid only worse. You still feel overpowered and somehow underpowered at the same time though.
The Item Drop system. Since there's so many customization options, there also a lot more class specific items. It also seems like the drop pool is specifically honed in towards your character. I very rarely ever found items that I was unable to use. It seems like when other players of different class are nearby, that drop pool expands to both classes. I have no idea how this will work in parties with lots of people but this is the impression I got from it. It's very strange. Since you can salvage items for crafting materials though, it's not like anything you pick up is worthless. Heck you can rip bonuses off of items you otherwise might not be able to use and convert them into upgradeables on weapons you can use which is neat. However, I'm just confused as to how exactly this will affect party play.
Level curving and scaling. I'm also concerned how the areas level curve with you will affect party play. Do other players just have to fight the same level monsters you do? Any clarification on this would be appreciated.
Diablo 3 Bosses. It looks like the Diablo 3 and Immortal boss monster mechanics are back for better or worse. I didn't mind them as much here. The ones I played were fine-ish but I feel like the "Set traps", "Fire projectiles", "Summon minions" mechanics will get stale very quickly if they abuse them as much as they already have. They also like to teleport on top of you a lot. I didn't find these fun in D3, and I'm not the biggest fan here. We'll see how they fare down the line.
Bloat. There's A LOT of features here that don't feel necessary and it's all overall very inconsequential. There's mini-events on the maps, tons of mini-quests, and the maps themselves are massive. The craft system is super generic. I don't understand the point of the Gambling System. Why not just use Gold? Why do mini events need their own currency? There's so much stuff to do here and it just feels too out of focus. It's easy to get lost among a myriad of ultimately pointless crap. I think this what will hurt replayability in the long run. Who wants to go through all this stuff?
Graphics. I won't harp on this too much because I'll happily sacrifice good graphics for cross platform play on EVERY device, but they're pretty bad. It's very much a trade off of Game Accessibility for Appearance. The biggest problem with graphics for me though is enemy design so far feels very uninspired. The best enemies I've encountered were the Treehulks who looked pretty cool and decently designed. The Phantom Wraiths weren't bad either. Reminded me of little Mini Mephisto's. As others have stated, the Fallen design can eat a dick.
One last thing before I get to the Story. Fuck the Spiders. They're the only enemy that almost killed me because they swarm you super quickly and spawn right on top of you. As such, they're my favorite enemies in the game.