MUD: "Discworld"

+ most detailed game you've ever played
+ 11+ years of dedicated staff and players
+ accurate and honest to the novels
+ massive and completely non-linear areas, ever-expanding
+ funny, clever, thrilling, a complete world of experience; takes literally years to explore (if this can ever be done)
+ best skills, PK, combat, roleplay, quest etc system you are likely to ever experience
+ implementation is superb
+ skills run into the "infinite"
+ heavily centred around "player skill", although at higher levels "character skill" cannot be combatted
+ design allows for endless strategy

- run purely by volunteers, which from both a game and moderation standpoint causes innumerable issues (some classes get more attention than others, moderators can delete you if you annoy them, etc etc)
- set in the Discworld so "nonmagical" skills are often mundane (warriors can "bash", "impale" etc)
- has been known to lag, perhaps fixed now
- small population with huge amounts of elitism/favourtism throughout and between players and coders (although no "cheating")
- players are overprotective of "their game" to the point of being rediculously closed-minded, as such suggestions for improvement are often met with spellbinding idiocy
- combat system has been slowed down

Summary:
This is "The" ultimate game if you're looking for immersion, interactivity, gameplay, social, detail, playerkilling, roleplaying, complexity, playability... no other MUD nor game has ever come close. But beware: if you're a griefer or like to swear in public, or be rude or generally let yourself go, you might find yourself losing your character after 500 or even 5000 hours of gameplay.