Bad UI example: the signup process.
I noticed this after viewing a link about twitter clones. If I want to sign up for a twitter account, this is the homepage. The question I have is: "How do I sign up?". Not in a figurative sense, but a real one... there's no 'Create an Account' right next to the login form, which has been a UI convention for some time now.
Stared at it long enough yet? I'll give you a clue: it's the big green box that says 'Join the Conversation'.
All told, this is a minor irritation, but at this point a user that is irritated by some interface quirk before they even sign up is going to wonder what the rest of the platform looks like: after all, user annoyances matter.