Highlights (Although the original list has since disappeared from jkontherun, ZDNet managed to snag the info):

•Built-in Hyper-V hypervisor
•Event logging support in SPC
•Fixes for DRM issues from WMP upgrades
•Windows Vista Feature Pack for Wireless
•Functionality to reduce resources required for sidebar gadgets
•Improved power settings for Windows Server 2008

The complete list of features:

Emerging Hardware Support

•SP2 contains Blue tooth 2.1 feature pack supporting the most recent specification for Blue tooth technology
•Ability to record data on Blu-Ray media,
•Adds Windows Connect Now (WCN) Wi-Fi Configuration to Windows Vista SP2,
•exFAT file system now supports UTC timestamps, which enables correct file synchronisation across time zones.
•SP2 provides support for new form factors, such as ICCD/CCID. new form factor support -example USB form factor as opposed to PCMCIA).
•Support for the new VIA 64-bit CPU
Security

•SP2 includes all previously released security updates, and builds on the proven security benefits of Windows Vista
•Secure Development Lifecycle process updates, where we identify the root cause of each security bulletin and improve our internal tools to eliminate code patterns that could lead to future vulnerabilities
•Reliability
•SP2 addresses previously released reliability updates, as well as addressing crashes, caused by Microsoft code, discovered since the launch of SP1
Performance

•Resume performance when Wi-Fi connection is no longer available after resume from sleep
•Inclusion of Windows Search 4 for improved indexing performance, improved relevancy in search, broader indexing scenario inclusion, as well as new Group Policy integration for Windows Search,
•Improvements to the RSS feeds sidebar gadget to improve update performance and responsiveness
Application Compatibility

•It is our goal that applications that run on the Windows Vista Operating System today and are written using public APIs will continue to work as designed on Windows Vista SP2.
•Previously released Application Compatibility updates are included in Windows Vista SP2.
•Spysweeper and ZoneAlarm now working with POP3 email accounts
Administration and Support Improvements

•Customers installing .net framework 3.5 service pack 1 will notice shorter download and installation times with Vista service pack 2 or Windows 2008 service pack 2 already installed,
•Service Pack Clean up tool (Compcln.exe): This tool helps restore the hard disk space by permanently deleting the previous versions of the files (RTM & SP1) that are being serviced by Service Pack 2.
•Single installer for both Vista & Server 2008
•Ability to detect an incompatible driver and block service pack installation or warn users of any loss of functionality
•Better error handling and providing more descriptive error messages where possible
•Better manageability through logging in system event log
•Componentization for Serviceability of the installer
Some Specific Fixes/Additions Include:

•Inclusion of Hyper-V
•Event logging support in SPC
•DNS Server now listens over ISATAP address
•Fixes DRM issues from WMP upgrades
•Windows Vista Feature Pack for Wireless
•Reduction of resources required for sidebar gadgets
•Improved power settings for WS08

It is not a mind-blowing upgrade, but there are definitely some useful tweaks in there. If you want to play around with it before it is expected to ship this April, Microsoft will make the Windows Vista and Windows Server 2008 Service Pack 2 Beta available to everyone through a Customer Preview Program (CPP) starting on December 4th via TechNet. [Jkontherun via ZDNet and The Windows Team Blog]