Microsoft have created a "Book of Longhorn" that covers all the changes in functionality from Windows Server 2003 with SP1 to Windows Server 2008.  I really recommend having a look at this one...

http://www.microsoft.com/downloads/details.aspx?familyid=173E6E9B-4D3E-4FD4-A2CF-73684FA46B60&displaylang=en