We don't generally add new "features" in new service packs. And because of that, we don't update the What's New guide after the initial release of the software. Service packs are mostly for bug fixes. You can get a list of the fixed bugs in regular service packs here:
There is a link to the above page on the download page where you get the service packs. Specifically for "point" releases (SP2.1 as opposed to SP2.0), which are fairly unusual, we put the information directly on the download page in the Technical Alerts section. Here is a link to the downloads page:
Reading that page, you can see that the only thing fixed in SP2.1 above and beyond what was fixed in SP2.0 and earlier releases is SPR#862458.
Also note that if there ARE any user interface changes or other major changes introduced in a service pack, those are documented in the Release Notes which are accessible via the help menu in the software by selecting "Release Notes", which takes you to this the following page, which is also accessible from the Technical Support section of our web page:
I hope this helps,