I signed up for Quora today to check it out. I'm not liking the un-threaded view of the stacked questions/answers. Of course I haven't spent enough time to give the overall UI a fair verdict.
One thing iI think they did right though, is that if you create an account using Twitter/FB or Facebook ID, right away it shows you questions from your existing Twitter/Facebook and Facebook friends, Soso you don't feel like a stranger to a new community. This is great for new users.