An official set of release notes accompanying the update says fixes are included for:
# AFP, SMB/CIFS, NFS and FTP network file services
# AirPort and Bluetooth wireless access
# Core Graphics, Core Audio, Core Image, and RAW camera support
# disc recording when creating and burning media
# .Mac sync services
# Spotlight indexing and searching
# Dashboard widgets: Dictionary, Flight Tracker, Stickies, and Unit Converter
# Address Book, AppleScript, Automator, Dictionary, Font Book, iCal, iSync, Mail, and Safari applications
# Disk Utility, Keychain Access, Migration Assistant, and Software Update
# compatibility with third party applications and devices
# previous standalone security updates