I would say:
- Improvements to OWA.
- Added multi-user email searching capabilities
- Improved IO utilization
- Added Instant Messaging functionality
- Added features like voicemail preview to Exchange 2007 Unified Messaging
Also I read article about the huge improve in the availability solutions in EX 2010 / not more increase in administrative overhead in creating Clustering solutions . i didn`t look at this features yet but seem promising .
The archiving feature is something most shops would be interesting to see and test in our “sandbox” environment.