I've played Avatar MUD for nearly three years now, and I've not yet seen a MUD to match it. In the years I've played, I've heroed several alts and gotten one to Lord status. Avatar does have some weaknesses which have pushed me into trying other muds over that time, but I always keep coming back. Avatar's three biggest strengths are the players (including the immortal staff, but definitely not limited to them), the balance and diversity of its classes, and overall friendly atmosphere of playing here.
The no-player-kill policy of the mud is a big step in promoting that atmosphere. If your idea of a fun time is summoning newbies into aggie high-level areas or otherwise ambushing those weaker than yourself, you'll find that a lot of your activities are forbidden by the game structure itself here, and the imms will be rather displeased with you in short order. If you find yourself at odds with this policy often enough, you'll eventually find yourself banned from playing (as has happened to even a few well-connected and highly visible players). But those who remain are those who are the type willing to lend a helping hand to their fellow player, and no other mud I've been to has been so successful in collecting such a player base.
An earlier post today made a comment that there were only two classes worth playing on the mud. I find that statement simply ridiculous. I've played nearly every class in Avatar, and like most of them. (My personal exception is the psi class.) The Avatar staff maintains a good diversity of classes along with a decent balance between the classes.
Gear is the overriding factor of how well an individual does playing in Avatar, and those few who have twinky gear will end up doing slightly better than the majority who have decent, good gear. But the players who have poor gear will have a tough time even gaining experience. Fortunately, a player sitting in the donation room can usually get most of the gear he'd need in just a few hours of waiting. And plenty of helpful players will help a new player get the gear he needs.
With that said, the weaknesses of Avatar should also be addressed. The biggest of them is that Avatar is indeed a hack-and-slash mud. While there are mini-quests scatered about, playing in Avatar often does boil down to find a mob and kill it. But I've logged in to Avatar simply to chat with friends, too.
The same kind of weakness applies to the role playing aspect of the mud. While role playing is offically encouraged, very few actually go to the bother of creating different characters for their alts. But again, what you put in to playing is often what you get back. If you play strictly by the numbers, you may level faster, but if you're anything like me, you'll end up having less fun.
And fun is the name of the game. And Avatar succeeds best in being fun.