• All client connections are now handled by the Client Access Server (CAS) - Including all MAPI connections
• New HA functionality for Mailbox Servers called Database Availability Group (DAG) - Replicates a mailbox to a maximum of 16 mailbox servers
• New wizard to create Certificate requests for Exchange (this is a good one folks!)
• Improved client experience through MailTips (warning messages to users BEFORE messages are sent) and conversation views.
• Integration with SMS/Text messaging
• Major improvements to database performance
• Introducing the Exchange Control Panel (allowing administrators to manage organization-wide tasks from a web-based interface and enabling users to manage personal settings / troubleshoot client issues)
• and many, many more...

I've been fortunate enough to play around with Exchange Server 2010 for the past year and I just think its a stellar feature packed product. If we can have a solid migration path from Exchange Server 2003 and Exchange Server 2007, we've got a winner!!!