I thought this was a great article that Technet published.  My favorites?  Continuous Availability via DAGs and one they don’t mention, the ability to have some users internal and some hosted in the “cloud” ….seamlessly.

http://blogs.technet.com/uc/archive/2010/01/21/top-ten-reasons-to-move-to-exchange-server-2010-in-2010.aspx