This is not going to be about bug or any past problems that have been addressed. This is about core, fundamental aspects of the game. I am well over 100 hours played and have basically beaten the game for what it has to offer at the moment. This will be a flat out list to keep things as short as possible.
(again keeping bugs/crashes/broken things that the devs already know out of this)

Campaign:
1: Play through was awesome. Everything felt, looked and sounded great. The dialogue's superb.
2: Ending was great all the way up to 'hey its been six months' and everything is normal. It didn't feel like a cliff hanger. It felt unfinished.
3: Playing with a staff is slow and frustrating up to lvl 20ish. Didn't get a upgrade on weapon until after killing the fire svriir.

End game:
A: Again visually and audibly everything is great.
B: For a good amount of hours you really feel like you are growing and progressing as a champ but then you just hit a wall. (when this happens varies depending on build)
C: A casual player will enjoy this game for weeks to months which is awesome. But gamers run out of steam quite quickly upon realizing how pigeon holed they are in builds.

1: Arguably the biggest issue is lack of clarity. The lack of easily understanding what spell/ability/attack/projectile does what type of damage material/physical/elemental/occult etc. is what is causing a lot of your players frustrations. We don't know what is increasing what and what affixes are increasing what abilities. There are too many different words and phrases used throughout your game and it's various skill/talent/attribute pages to really make any sense of it all. Especially when you have multiple ailments and your spells/attacks say a different ailment every time you hover over it. Sometimes they all go off sometimes you're missing two or three.
2: Inventory page needs on the top left of your gear slots on your champion What elements/ailments for attacks AND spells you have. This will show you how much you need to swap ailments on attacks/spells because this is currently a crap shoot trying to figure it all out. Which, again, is still very convoluted.
- Attribute page needs much more clarity on "resource" gained actively and passively for both.
3: The absolute vast majority (including me because I can't find much of anything about this) of your player base has absolutely no clue about all the different possible affixes on gear. I spent every gear rerolling item I had trying to get aether on items which now I'm certain some it's not a possibility. Something this complex and with 3 layers on the gear (hp/shield/all res., white attribute stat & 'X' amount of affixes) needs to be shown somewhere on some list so that we don't spend 2 weeks worth of rerolling reagents on stuff that isn't even a possibility.
4: Attacks/spells are horribly balanced in terms of damage/utility/output/viability (even after bleeding edge nerf). This is due to the intertwining of abilities, ability modifiers and the passive tree.
- Each ability has 16 modifiers yet most are dull and it's hard to notice whether you have them chosen or not. +20% base damage is not woth much in any capacity yet these little droplets of damage or minor changes like a '+1meter' increase are negligible in most cases and that's even if its viable to take. When they do actually alter the ability most of the time it's a one sentence description leaving you at a loss.
- Almost half of your entire passive tree revolves around ailments. (YOU ARE OBSESSED WITH AILMENTS.... STOP!) Ailments that are extremely hard to monitor if you have them at all, if they are working or even if they are on the enemies and how many stacks they have or you have. This pigeon holes many builds to go the ailment route. This completely negates certain abilities due to the ailment incompatibility.
-i.e.: A 2 handed 'warrior' has 1 frost modifier, 1 stasis modifier that's broken still, 1 lightning (that I can't seem to make viable), 2 sacred and 5 fire modifiers (FIRE IS EVERYWHERE IN YOUR GAME AND ON GEAR. ITS THE #1 ELEMENT THAT POPS UP BY FAR). You can't get the rest or its insanely troublesome due to above reasons. builds pretty much make it impossible to get aether damage and several other things that are throughout the passive tree.
- If you take the passive "gods amongt men" making you pure material damage you literally don't have enough in the passive tree to use all your points in wisely. On top of that it's really not even viable. The multipliers are not there and neither are the abilities. This is also true for several other aspects of the game.
- When you choose a ability or item and then change any one thing about it; that change can break literally everything else you have and you'd have no clue why. This is a incomplete design.
- There is no guide. You'd need to play around with god knows how many pieces of gear for a absurd amount of time. Simply changing from 1 element to the next often times requires ENTIRE new set of gear, gems and weapon.
- drop rates for gems and reagents needs to go way up.
- It's out right currently a frustrating pain in the butt to switch specs to keep the game fun. It requires gear sets, weapons, abilities, modifiers, passives, gems. All of which can break whatever it is you are trying to create for almost no known reason. (It seemed that you tried to use the most frustrating part of Path of Exile)
-Things revolving around abilities, modifiers, passives and gear that are not clear and concise are leading to discontent and confusion.

Complexity and the unknown does not make a game fun. This game needs simplification and clarity to some degree