I have played MUME since 1995, on and off. After trying out a number of MUDs, I settled for the kind that is 100% original areas, mobiles etc. MUME has this, plus the ability of playing both good and evil in the War of The Ring.
The game is huge (almost 20k rooms) with areas like Tharbad, Moria, Rivendell, Eregion, Grey Havens and many more, which leaves a lot of room for exploration. The areas are generally very well built, and have tons of special effects coded in Mudlle (an internal quite advanced programming language), enabling mobiles to be quite "smart" and really improves the gaming experience.
In conclusion I would recommend this MUD for explorers, level-racers and PKers alike - since it has all of it.