The best way to check out GNOME 3's new features -- and it has lots of new features -- is to run a live version of openSUSE or Fedora, or simply head over to the GNOME 3 website and watch the (rather pretty) introductory videos. If you want a synopsis, though, here it is: GNOME 3 looks a lot like Mac OS X, with a healthy dollop of iOSesqueness for good measure, but yet it still somehow retains an underlying feel of Linux.
The overall aesthetic is very simple, very elegant, and despite being slightly out of fashion, there are plenty of rounded corners, too. The main addition, workflow-wise, is the addition of an app-launcher-cum-alt-tab screen, where you can launch apps, or flip through your open windows. For a complete list of the new features and changes, check the GNOME 3 release notes.
Despite GNOME 3 being officially launched, there aren't actually any releases for existing, stable Linux distros -- it's the live CD/USB images, or Ubuntu users will have to wait for the launch of 11.04 for a GNOME 3 PPA, but it will break Unity in the process. Fedora users will have to wait for for the May 24 release of Fedora 15. Of course, if you're feeling crazy, you can always build GNOME 3 from source.