I recently installed the beta Windows Server R2 release, which comes with WSS SP2.
You can find the overview of the new features here.
The first thing you will notice that is different when you install WSS R2, is the option to use NTLM or Kerberos:
Also, you will notice that the site versions are different that those of previous versions. The following is a comparison of WSS with no service pack and WSS SP2:
More info to come as I have time.