Usability in its wake, takes efficiency into its full height, hence it dictates that the best path is the one requires least clicks, hyperlinks should be in blue and underlined, etc.; it puts heavy weight on conformity and very little on aesthetic. The best design according to Usability thus, is the one that’s functional; often sterile, and boring.
User Experience on the other hand, still relies its ground on Usability, but only for those parts that requires it to be. On the grander scale however, it puts its focus upon a different kind of “god”: user, taking into accounts its human characteristic; which often have a healthy dose of subjectivism, emotions, irrationalism, and doesn’t always aim for what’s simply usable.
Therefore for its characteristics, I think Usability is like the early-Modernism, while UX is the late-Modernism — not Post Modernism though. (byms)