Yea, generally I prefer MUDS with some sort of developed UI. In this aspect, hellmoo was brilliant, making excelent use of ASCII maps allowing you to have instant general idea of where you are and whats around you. Wayfar also does a pretty good job at this.
But yea, I prefer muds with in depth character progression that allow you to create characters that play completely different from one another, rather then just a basic d&d class thing. In this aspect hellmoo also shined, since there were mutations, and some mutation required others to become attainable. This eventualy culminated in mutation paths that radically changed how you played and your standing in the game world.
A vampire had tons of extra stats but literally burst into flames in sunlight, forcing players to adapt to going out during the night, staying indoors and using the sewers to move around during the day. Abominations were actively hunted by pretty much everyone, meaning you cant interact with civilization anymore (including shop npcs) and forcing you to adapt to living in the wilderness and being quick and furtive in your urban incursions (or just being strong enough to lolsmash anything). Zombies would literally rise from the dead a little while after being killed, if their bodies remained whole, etc.
I want a MUD that lets me do crazy things with my characters, not just become better at killing stuff using A, B or C method, like teleporting around, becoming invisible, mind control, eating other players, hunting and being hunted by npcs/players, transforming my character into other things, losing limbs and growing new ones. I also want a MUD that allows me to create my own stuff, my own buildings, etc.
A proceduraly generated world ALA wayfar is also a plus, since I want to be able to explore around and find stuff that nobody else knows.
Yea, I know, I'm pretty picky, but I dont see a reason to waste time on a MUD if it doesnt at least give me more freedom then MMOs do.