The actual UI isn't what I'm interested in so much - frankly, Metro, Windows phone 7 and 8 both work fine strictly as the basis for UI's - but rather, I'm interested in what's going on with control and access... Mainly, the file system, and whether you can access data through syncing or a proper file system. This will be telling for MS's future plan - do they want to go for a platform where they control everything like iOS, or a platform that gives substantially more access to what's under the hood, like Android and proper Windows.
I VERY much hope for the latter, as I will NOT be getting another Windows phone without more free access to the file system and less of this syncing BS.