I feel that the arrows that select the Action Bar are way too small. I find myself requiring a few clicks to hit them when playing on a 24" screen. It would be nice if they were a bit wider and just in general a bit larger.
This isn't a huge problem, mostly just a niggling annoyance. There are the keyboard shortcuts, but sometimes I just prefer to play with just the mouse if i'm feeling very lazy, or i have a venerable feline on my lap.
I was going to also mention inventory sorting, but they just added that, so good job Larian!