I've noticed a few key issues that are really bogging down my experience.
1) Random lag on zoning, requires an alt tab to fix
2) Alt tab requires 2 attempts to reenter the game, every time, without fail
3) Completely random crash to desktops, with no error, and no warning.
4) erratic AIs - Summoned pets will often run into walls, and mercs/pets will stand in doorways with no way to get past them.
5) too-random dragons, i appreciate the randomness of dragons.. but them being able to pull an about face while flying at full speed, and often flying circles around you for a good minute or two without doing anything, doesnt make sense.
6) compatibility with steam svckS, if you are going to release a game on a platform.. optimize it

few others that arent at the top of my head right now.. but those are the big ones.