No directly related to unresponsiveness but still UI:

There are many dialogs using scroll bars and drop down menus (combo boxes), this is not OK for stuff that you do 100 times a day.

The crafting dialog is the best example, with the crafting abilities forcing you to scroll down below 'wait' to select them every single time.

Also the quest journal dialog, the combo box quest type filtering could be easily be replaced by toggle buttons, tabbed filtering, or any other one-click-away UI strategy.