AFAIK, the servers did not practically change in MC13, but it has lots of new and bettered small things under the hood.
Personally, I didn't upgrade my main "production" database (which we use on our home LAN) from MC11.1 to MC12 during the two year development cycle of MC12 -- despite the fact that I installed and beta tested about five hundred MC12 builds. I installed many of the tested builds on more than one PC so the total number of installs was actually more. In general, I liked MC12, but it had always a few small things here and there that were not quite right for me and I always postponed the move. Most of these things were fixed or bettered in MC13 and it is now my preferred version.
A link to the complete MC13 change log is in my signature.