One of the really nice things I found about MUDs was how surprisingly accessible they were for creating your own content. I taught myself to program on an LPMUD, and the multi user nature of MUDs taught me a fair bit about optimising code, even for simple class abilities for characters, since they were typically run on underpowered boxes that people had brought in and dumped on a university network, and when players find out that some times can make the server chug for everyone else by using abilities, they will and encourage others to do so as well :)

Uni taught me a lot about the theory of the things that I'd learned in practice previously through MUDs.