Amen on the minamalistic UI. I like I UI that is crisp clean lines that you can move the peices around to edge one another, and forget they are there with the exception of the odd glance. Fluffy flurrled artistic edges that don't line up worth a flip are nice for the first few minutes, but then you ave to orginize the ui in a way that you can make sesne of the inmormation presented. It's kinda hard to do that if the pieces are taking up so much real estate you can't see the world, or other peices that may be more important than the one your filldling with.
Having not played WoW i wouldn't know about the add-ons, but it sounds nice. Someone mentioned maps in previous post. Perhaps an interface element that will allow you to trade maps with others you come across online. or import export map notations from player to player within the game via a system like item trade. Of course you'd have to have the ingame option to actually scribe the map you have for your personal use down into a ingame item, otherwise you would turn VG into a glorified file sharing program.