It's not difficult to learn. Lots of small quirks that are unfamiliar at first and several UI features like Expose (Think it's called Mission Control in 10.7 Lion. Haven't upgraded to Lion yet) and Spaces (virtual desktops), but nothing that you can't figure out on your own.
A few things that are unfamiliar to Windows users:
- Closing a window (clicking the red x in the top left corner of the window, or pressing cmd+w) does not quit the application it belongs to. If you want to actually quit an application, hit cmd+q.
- There are no "OK" or "Apply" or "Cancel" buttons on preference windows. If you change a preference, it takes effect immediately.
- cmd + alt + esc opens the "force quit application" menu, allowing you to quit...