I've tried a number of MUD's, and while the combat in most was rather similar, Discworld distinguished itself in a different way: Long room descriptions, and practically anything in a room description can be looked at too. Also objects appear to handle like in real life. If you drop or even bury something, it will be there (at least for some time) later. Also spells are not just the lame "The wizard casts a fireball you" but more like five lines of mystical actions the wizard performs, just like an onlooker would see.
Also, the quests are not so straightforward like get me this, or kill that, but are embedded in the whole enviroment, and may take a few steps. Often you are just curious about a new area, and enjoying the descriptions and find out you completed one of the few hundred quests.
All in all, it is a beautiful world, held together by the discworld theme, where you can even get your experience by exploring, just casting spells to help others, or even idling and just chatting.