As long as we're plugging MUDs here, there's one called Geas (
http://www.geas.de/) that I've been playing on and off for the last couple of years. It's got an amazing level of detail and is roleplay-enforced, though unfortunately none of my characters have ever gotten very far, due to me getting bored with only rarely seeing other players on during the times I can play. And of course the few I did run across were all wrapped up in some RP drama that went completely over my head.
In fact I often do feel out of depth in roleplaying situations - I just can't seem to think fast enough to decide how my character would react and then do all that emoting, and I get sick of everyone being so ultra serious all the time. It's especially unfortunate because as others have stated, that kind of game are great for keeping the more immature folks away, and I do like the idea of roleplaying, I'm just terrible at it in a 'chatroom' like setting. (I wish more people were still up for play-by-email games, that's where I got started at and is still what I prefer...)
The only thing I've ever found that seemed practically made for me is a browser-based game called Cantr. (http://www.cantr.net/) It's a slow-paced, roleplay-enforced game that you can play for just five minutes a day if you want, and I've honestly never seen anything remotely like it. There's no magic, gold coins, convoluted backstory or anything like that - your characters (you can have up to 15) are just normal people who live, roleplay and work (work projects continue even when you're offline) with others in whatever place they decide to settle in. The cities themselves are built by players who gathered the resources and made the tools to put up the buildings, create their own system of government and laws (and ways to enforce them), build ships and other vehicles to explore, set up trade, etc.
It's not for everybody, but if you're looking for something different you might give it a try. It only has two major flaws - 1.) the player base is much too small for the size of the world (and most of the players are Polish, living on the Polish continent, though their explorers are having more and more contact with English-speaking players lately), and 2) the staff seems bound and determined to stop characters from making any progress and slowly suck all the fun out of the game in the name of 'realism'.