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There is a list of notable changes [technet.microsoft.com] in the service pack. Few of them effect my use of the OS, and I can only say that I've noticed dramatically better performance in the system search. Everything seems a lot faster than before. Other than that, SP2 hasn't made much of an impression on me.
If you're using Windows Server 2008 then you can look forward to Hyper-V virtualization will be fully integrated.