New implementations of iCloud will let you stream purchased songs from the cloud even if you haven't enabled iTunes Match, and let you pick up movies and TV Shows where you left off on other iCloud-capable devices.
Our favorite part of the update is the new mini-player. It's smaller than ever, eats up less than half the system resources of the full version (because, alas, iTunes 11 is still a bit of a resource hog) and new drop-downs for search and up-next songs mean you won't have to revert to the full window nearly as often.
To get the update, open the Mac App Store or run Apple Software Update on your PC. Or just download it here.