Exile is the first MUD i've ever played, and probably the only one that i didn't stop playing after 10 minutes. It's got a well-done quest and pk system, and players that are wonderful. The amount of people who play aren't so small that you feel that you know everyone's life story, yet not so big that you feel like you're overwhelmed.

It's pretty well balanced, melee + spellcaster classes, and almost everyone's always willing to help the newbies, imms and heroes included. The clan/tribe system's good too, and as the game encourages being in a form to xp, you're gonna have at least some, more like a lot of, interaction between players.

I dunno, you've all probably heard all of this stuff before in reviews, but come try it :). it's worth it.