It will be available for download on MSDN and TechNet on February 16, and on Windows Update on February 22.
The two main new features on the server side are Dynamic Memory and RemoteFX. Dynamic Memory gives you more flexibility in Hyper-V with how you configure memory settings to get better use out of your available memory and increase VM density on the host.
RemoteFX lets you use the GPU on the host Server to handle some of the graphical processing workload for remote clients. Personally, I am curious how well this works with the typical low-end embedded GPU in most commodity servers. I also wonder whether you could install a higher end, discrete video card and have RemoteFX use it?