I think they've struck on something that Android has been ignoring: fun. The original iPhone was full of fun, interesting usability tidbits - the momentum-based flicking, the bounce you get when you hit the bottom of a list, the turning of album covers... etc.
How "fun" your UI is directly contributes to discoverability. Android's UI is very dry, and IMHO it makes the UI harder for a novice to pick up and learn. You can inject a lot more features that aren't immediately obvious, and must be learned - but you must make your platform "fun" enough for users to actually bother discovering these things.
Given what I've seen so far, I think MS has realized this also, and I think they're on the right track.
But did we forget all the lessons of the "readwriteweb facebook login debacle" of last week?
This UI seems substantially less user friendly than iPhone or Android for your average end user.
Who is now, incidentally, trained on the "iphone clone" UI if they've had a smartphone in this generation.