From what I've read so far:

• > 1000 changes and bugfixes for ZFS. TRIM is now fully supported and there should be a performance boost.
  
• Improvements for UFS, especially a series of bugfixes for journaling. 30% speed increase for fsck on newly created UFS volumes.
  
• NFS has undergone some server- and clientside modifications. It interacts better with ZFS and is faster overall.
  
• Unmapped IO should improve IO massively. (Can't really tell anything on that ..)
  
• Clang updated to version 3.3, so was libc++. FreeBSD now supports C++11
  
• KMS / GEM for Intel GPUs now works better.
  

But you seem right, I was also not capable of finding an "official" feature list. Important bit from the announcement:

Quote:Important note to freebsd-update(8) users: Due to a last minute problem
found in the 9.2-BETA1 freebsd-update(8) builds, freebsd-update(8) is
NOT supported for 9.2-BETA1 upgrades. Please do not use
freebsd-update(8) to upgrade to 9.2-BETA1.