As John Howard mentions in his post Windows Server R2 is entirely based on Windows Server SP1 - it simply adds features and includes some code updates. The design goal of R2 was to bring additional features without changing the Kernel so as to ensure maximum application compatibility and minimal integration testing cycles for you.

There are three virtual labs you can access for free to enable you to try the new features for yourself right from your browser - they cover Active Directory Federation Services (ADFS), Distributed File System (DFS) used for a branch office application and Efficient Storage Management - just browse here to give them a whirl