What I loved about the MUD as a learning environment was the players. On a busy night we'd have over a hundred people playing. So, I got to cut my teeth on a real, live production system with actual users. That motivated me. There were mild consequences if I broke things. And, if I made things better for the players, it felt good.
For me, this environment was so much better than doing programming problem sets by myself, writing code that no one would ever use.