XP to Vista wasn't really a major change in terms of user interaction. They added UAC, which might be annoying, but followed existing interaction methods, so it was pretty obvious how to interact with it. Aero prettied up the UI, but the interaction wasn't actually changed by it. Snap added some new interactions, but the old actions were unchanged.
What do you see as the changes that forced the user to "re-learn the OS"?
What do you see as the changes that forced the user to "re-learn the OS"?