First off, bump this, 8 slots is not close to beeing enough.
With all things considered, they got really cool looking and acting spells that also work together, this alone is really good but when you got around 20-30 skillpoints you end up with way to many abilities left unused.
The second problem we run in to is the "Must have", i notice someone naming KE as a good example were you can cancel barrier. but to mention a few; atleast 1 heroic ability, all the quick move abilities to get out of fire like fadestep, archer leapingshot, rogue stealth, etc. this ends up beeing less then 8slots due to the almost mandatory ability requirements. (Im not saying you HAVE to take them or use them, all im saying it makes things alot more challengering then it have to be if unless your in to that type of thing, but then you intentionally limit YOURSELF, instead of the game limiting you forcefully)
Then there is the lack of attribute distribution. the place of leveling up where YOU the player get to decide what you want, and what is useful. the game might point you to magic as a mage, or strenght as a warrior but you can chose to ignore it if you want. What I mean is that alot of the passives we get gives you the attributes for simplicity, but instead it should give the options to have a quick level up that sort it out for you or something in those lines - aka simple level system or advanced.
What the game has done well, i think atleast is the combo and synergy abilities have. they add wow factor through looks and through big numbers, efficiency and so on.
So what can DAI learn from this, go back to your roots, this game is very good at the end of the day (after some patching etc.) but still has work to do, like adding more skillslots (atleast twice as many)
offtopic: better AI, better tactical cam (aim to not limit PC because of consol, but rather max out both of them so consols get the best they can get, and PC get the best they can - if they can) and probably more but im sure others will fill you in on that 