Other than rolls up all previous released windows patches since RTM, Windows Server 2008 R2 SP1 contains two new features. Dynamic Memory and RemoteFX.
Dynamic Memory is a memory management feature for Hyper-V VM administrators. Dynamic Memory allows administrator allocates a range of memory to indivual VMs and windows system will automatically adjust memory of each VM based on performance.
RemoteFX creates a local like experience for remote users, including windows Aero and 3D contents.
You can read more about SP1 from Windows 7 Team Blog