There's a lot of welcome changes in 7, almost too many to list
Some of my favourites:
* Delayed services on start-up (no more hard drive thrashing and waiting 10-15 min until Windows settles down)
* Explorer Window>Right-click>open new Window (long overdue)
* Aero Snap/peek/live preview functions
* Start menu - Easier to find system tools by typing in the search field ie type 'create' and you have an option for creating a restore point. Doesn't exist in Vista etc
* Changing DPI without restart
* Libraries (I like them

)
* Plus heaps more