First I want to say I like what Anet did with Achievement Panel UI in the hero tab.
Now Anet need to work on the rest of the UI. Take the Event assistant We need a open way to keep parts of it open of closed. So have a cleaner UI in the right corner of the screen if we want.
Messages popping up in the middle of the UI while playing is also a problem. How about they pop up over the map instead.
Option to replace the map with Skyrim style radar.
Target display I feel is to large and often blocks view.
Gem store need a new UI too. There are too many new skins for the current gem store UI. It needs better organization.