Here is a good article listing the upcoming improvements in Exchange 2010 SP1 from the Exchange team at Microsoft:
http://msexchangeteam.com/archive/2010/04/05/454533.aspx
Key points include:
Featured enhancements:
- Archiving and discovery improvements
- Outlook Web Access Improvements
- Mobile user and Management improvements
- UI improvements
Plus also there is a rollup of all the existing fixes included