There are various enhancements and even changes in SP1. Perhaps one of the most interesting is the change in security: applications launched from LocalIntranet will get FullTrust by default from now on (read here). I guess this is because a lot of people found annoying that they couldn't launch applications (that required FullTrust) from network shares. This move will certainly ease the internal deployment but at the same time it might cause security problems, too.
Here is the list of .net 3.5 SP1 runtime performance enhancements and another list of improvements. A list of changes in TFS. Brad Abram's list and finally Scott Guthrie's take.
Download the bits from here.