Having played this MuD now for almost a year, and recently having been promoted to the Creatorial Administration, I have to say this is the best MuD I have found.
1) Lots of players. This really makes a MuD, and the discworld often has over 150 on at one time, with a playerbase of over 2,000.
2) Lots of unique rooms. These are not rooms just generated by a script, with the same crappy description, these are rooms that have been created with great care and attention. Stand still, and little things happen. Humour is around.
3) Frogs! Wombles! Fluffy Sera! [note: injoke. Join the mud to find out :)]