As of the SP1 release, Windows Server 2003 now supports access-based enumeration of file shares. Basically, files and folders to which users lack access will not be visible within file shares. No more double-clicking shared resources only to be greeted with “Access denied.” This is quite a nice feature, and one which is long overdue.

http://www.microsoft.com/windowsserver2003/techinfo/overview/abe.mspx

Also, the new release of Windows Server 2003 R2 appears to have a number of new features that were not included in SP1 (and that probably will not be featured in SP2). As far as I can figure out, R2 is related to 2003 SP1 as NT4 Option Pack was related to NT4.

http://www.microsoft.com/windowsserver2003/r2/whatsnewinr2.mspx