My two biggest problems are:

The time between hitting a button and having the action play out. Whether it's server related or game related, it drives me crazy and is the only big reason I don't play classes outside of crafting and gathering ones anymore.

Keyboard key bindings. I know this is being worked on, and I even have a work around with this extra little hand-sized keyboard of mine. But even so, I'd like any key on the keyboard to be fair game in mapping to some function. This includes mapping to keys that are on the other 0-9 bars. My usual setup is using 1-4, q, e, f, c and the middle mouse button for abilities. That is 9 keys, but combined with shift, control, and alt, that's room for 36 abilities. Each of which is accessible without taking my hand off the keyboard.

Just those two things would make me happy for a while.