Windows Service Packs are traditionally roll-up packages of hotfixes into a single, manageable package for easier updating of the OS to a known (minimum) state – they almost certainly contain extra changes that were not in any package delivered through Windows Update (LDRs) and occasionally other changes not present in any hotfix package.
Sometimes there are performance improvements (such as SP1 for Windows Vista, addressing file copying and network performance).
Rarely there are major feature overhauls or additions (such as SP3 for Windows XP, revamping and enabling the Windows Firewall by default).
Most of the time it is a collection of fixes for issues you never encountered (check the “list of updates included in this service pack” and you will see many LDR KB articles, I would imagine almost no individual has hit a significant number from this list).
SP1 for Windows NT 6.1 (7 / Server 2008 R2) does not yet have a release date, but there are 2 new features for Windows Server 2008 R2 affecting the area of Remote Desktop Services and Hyper-V which have been announced and you can read about here:
As a quick review of the Service Pack count for Windows NT since 4.0 up to today (it has been confirmed there will be no more Service Packs for Windows NT 5.2 and earlier):
NT 4.0 – 6
NT 5.0 (2000) – 4
NT 5.1 (XP) – 3
NT 5.2 (2003/XP x64) – 2
NT 6.0 (Vista) – 2
NT 6.0 (Server 2008) – 1