About Atlantis: I tried it, and it is pretty great. I was having some issues but they were resolved by checking their forums. Thanks!
... OK now that we have a recommended client, can anyone recommend any good MUDs?
If you want:
-intense roleplay (enforced)
-a detailed, original, fascinating fantasy setting
-challenging PK in a dangerous world
-deep mechanics (with, unfortunately, a steep learning curve)
-great IMMs and a dedicated player base
try avendar.com. I've been playing it for years, and it's better now than it's ever been.