Anything big that would make me want to upgrade from F11 to F12?
I don’t know about big, depends on your usage.
http://fedoraproject.org/wiki/Releases/12/FeatureList
I like the new optimization for newer x86 processors.
http://fedoraproject.org/wiki/Releases/11/FeatureList
I’m currently running F10, so I’m also looking at the new speedier boot process and KMS for graphics updates.
Some of those features you don’t realize they’re affecting you, but they do. In one way or another, most often times to the benefit.