I agree Hall . Most OS SP's don't include new features. As you said, XP did, but I think that is the only exception. My reason for that as a potential "answer" to my "What is a Service Pack" question was just that: A potential answer. Microsoft could include new features of the OS in a SP, but they have not done so (except as noted in XP).
Which is why they are good as a "one stop install of security updates" which in my mind boils down to installing one big patch instead of 100's of little ones.