better transparency and drop shadow support, a menu editor, clearlooks is default now, chiro (or however you spell it) rendering engine, better file manager, The panel looks better if mounted on teh side of the screen, CD ripper, Improved Web browser with more extentions, Evolution looks better, new control center dialogs, Mouse themes, the Evince document viewer (pdf and other file viewer, much better the adobe or gpdf imho), image viewer is a little better, help viewer now uses web browser, search now shows thumbnails, The dictionary is better looking, the improved gnome minesweeper 

, better system tools for controling services, a nice log viewer, gtk 2.8 improvments, more standards with freedesktop.org, a actual clipboard that works when you close apps, etc.
You can read all about it here 
http://www.gnome.org/start/2.12/notes/en/index.html
 
This makes me very excited.
The GTK+ library is already popular among developers who need to support multiple platforms, including Microsoft Windows as well as Linux and UNIX. For instance, artists may edit their images using GIMP and Inkscape on either Linux or Windows.
And now many more GNOME libraries, including ORBit2, libbonobo, libgnome, libbonoboui, libgnomeui and gnome-vfs, can build on Microsoft Windows, making it easier to build and distribute GNOME applications on that platform. While this support is not yet complete, it may be sufficient for some applications, and is expected to be complete in the next release of GTK+ and GNOME.