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"?

Example: You need to search some folder and it's subdirectories for files.

In WinXP you might do: Start->Find folders and files. Write search terms. Location Dropdown->"Browse..."->Find folder. Press search.

Vista: Find the folder in Explorer, write your search terms in the search box in the upper right corner .

This workflow wasn't changed for a long time, as far as I remember.