It doesn't look like they've released the full "About Server OS X 2.1.1 Update" document yet. Here's the one for v2.1 for eventual reference: http://support.apple.com/kb/HT5497
From the partial information from Software Update (in the Mac App Store), the one thing that stands out is the
retaining of network, DNS and PHP settings [when] installing or upgrading OS X Server.
Based on some threads here (and my own experience), that was a problem with the v2.1 update, where some of those setting where blown out on the upgrade. In my case the DNS settings were lost, and there were some other things that went wrong, e.g. Open Directory DB got hosed.
For the moment if you've got 2.1 installed already, I'd wait until getting further details about 2.1.1.