UAC is probably the last big one I can think of, so Windows 7*. The last few times I've had to do work on an XP machine it was the one feature I missed, invariably logged in as a standard user and realising I needed to switch into an admin account, rather than just run the installer and enter my admin creds when prompted. Powershell being universally available would be my second thing, but that doesn't come up as much when having to support old customer machines.
* I know it was released in Vista, but IMHO it wasn't polished until 7.