Since I just started in 0.6, I can’t give much feedback compared to previous versions (except the pre-alpha), so I’ll focus more on suggestions.
Campaign:
I like that it’s not called Act 1/2/3, etc. Much of the level design and layout could use help–it felt very same-y, and the pre-alpha was a more fun experience for me in this regard. Enemies usually ignored me until I was hitting them. Skills could target beyond line of sight, so I can hit enemies around corners, long distances away (I play on a TV), etc. There was a lack of variety in monster types, behaviors and attacks.
Skills:
This is my favorite part of the game–I’m so glad to see more skills have been implemented. There are a few minor bugs I’ve run into, but they were all to my benefit (Summon Skeleton+Deathly Ambition not removing my life on cast, etc). I am too excited to see some of the more advanced skills being added, even if it’s just a few nodes at a time and not the full tree. Please let me modify Aura of Decay 
Passives: It took a second to figure out how it exactly works, but once you figure it out, it’s solid. There wasn’t a time I didn’t realize what I needed. It was however difficult to keep track of quests that grant passives. I think I got 4 of them? I don’t have a good way to verify if I got them all or not. It’s also difficult to find where exactly these quests are, and sometimes I find them by accident.
UI: It looks and feels clean, especially compared to the pre-alpha. I would love to see more informational panels, for example, in a skill tree, if I could see a list of all the mods effecting a skill I have (6 Maximum Skeletons, You cannot summon skeleton warriors, 80% mana efficiency, 80% crit chance, etc), especially since so many of them stack, that would be one of the most useful things I can think of adding to the game. It would be nice if there was similar info about your gear listed in your inventory as another tab over your stats.
Pie in the sky, very later future, it would be nice to see a way to preview stats before you take a node, import/export skill/passive trees and share them, etc. I’m the kind of person who likes theorycrafting on the go, and the best I have is an outdated wiki right now.
I know many of my criticisms will be addressed in the future, and likely are on the roadmap, I’m only commenting on what I see right now.
I’ll add more feedback as it comes to me, or as I find more bugs/issues. Thanks for making what is the most exciting game to me in… years honestly. Keep up the good work, and for the love of all that’s good, please don’t turn into Wolcen. 
Edit:
Something else I ran into, when using the passive that generates a wraith on kill (25% chance), I popped a pack in Arena, and spawned 399 Wraiths at once. I didn’t kill 1200 mobs though, and it made me instantly drop to about 2fps. I’m sure optimizations are in the future. As much as I hate to say it, skills like that might need a cap.
Also, Arena, when you die, there’s only a respawn button–there should be a return to town option. I’ve been logging out to go back to town to safely allocate my passives.
Edit2:
While crafting when I open my screen, I often find it has defaulted to trying to craft a suffix/affix even if I haven’t put anything into the box. If I right click my shatters, they just disappear into the ether. I would love to see the various crafting items in the selectable list while you’re crafting. If an item is full of mods, those should be the only options that show when you drop the item into the crafting box–it’ll make the crafting process more enjoyable. I’ve been having a blast with it so far, and I can’t wait to make something epic 