It doesn't really matter which MMO or MU* you are taking part in.
Mostly RP is self generating.
What I mean is, if you stay IC, eventually you will attract others who want to RP. This is assuming you don't immigrate to a game with like minded friends.
The quality and style is HUUUUUUUUUUUUGELY dependent on where you are though. MUSHes have a very different style of RPing from MUCKs have a very different style from several different types of MUDs.
The key, really, is to surround yourself by people who like the same RP style that you like. Generally low-consequence and mostly player consent based, with little continuity? MUCKs. Playing your character to the freaking hilt and never letting anyone see the player behind the keyboard, even to the point of killing other characters? Some hardcore MUDs. Occasional light social RPing, accompanied by long forum posts, but no consequences at all? Other, social MUDs, also MMOs. Serious, realistic consequences RP that's often slightly consent-based but ultimately unpredictable and dangerous? MUSHes.
If you want plotlines that aren't invented by your fellow players, but instead have some staff support, you want either a MUSH or some of the harder-core MUDs, not MUCKs or MMOs. If letting your character die for the sake of a good story appeals, you want a MUSH, period. If your character's story is the ultimate important thing to you personally, you want a MUCK or a social MUD, where nothing's really out of your control. If you see roleplaying as potentially competitive, you want a hardcore MUD.
These are all pretty heavily enforced by the rules of the game. For reference, you can probably classify hardcore MUDs as ones where your character can actually die permadeath, and social ones where you can't, regardless of PK...so all MMOs I've ever seen were social MUDs.
MUCKs, I've hardly ever seen a MUCK where the staff got involved in the plots running on-game, so it's impossible to enforce any continuity. Which is great if you like to get into plots where you slay your first dragon (or 'slay' your first 'dragon') repeatedly with the same character, or decide that you didn't really like it when that old gray wizard turned your hair pink, so uh, I guess that didn't really happen, or you handwaved fixing it. Great for personal control over your character's fate. If it was meant to be permanent, you probably already agreed with the wizard OOC what would happen ahead of time. Many people insist in some continuity within a given interpersonal relationship, IE that wizard might be upset if your hair wasn't pink next time he saw you, but the player wouldn't care if you turned it off otherwise. Plots on MUCKs are frequently considered somewhat private because of this loose continuity, there's rarely real global stuff. The lack of consequences also mean there's not much need to talk out-of-character before running plots, while also moving some OOC banter into the game itself.
MUSHes, I've NEVER seen a MUSH where the staff -wasn't- involved in the plots running on-game, and the players on MUSHes are pretty hardcore about continuity. They'll talk, and be honestly very surprised if you take the result of some other plot totally unrelated to them and handwave what happened. OOC and IC are enormously regimented. If something horrible happens to your character like getting your arm chopped off, chances are everyone is going to want to hear the story about it, though, and are likely to console you OOC. Some MUSHes are more consent-based than others, but in the majority of them, you -would- know ahead of time that you were going into a dangerous situation, but you would -not- know exactly what would happen. MUSHes frequently make it possible to stay out of harm's way, but that usually means actually staying away from the interesting things going on.
Social MUDs, well, some of them have very elaborate social constructs but they are usually distinct from the actual gameplay. So at the end of the day, not much is going to stand in the way between you and slaying twenty giant rats for their tails. And there is pretty much never any real RP enforcement, you will always run into some people who just don't RP at all, and usually everyone has a different idea of how plots should be run--moreso than other places, I've found. You have a good amount of control over your character's personal story, but it's often hard to get other players onboard because the population is so very diverse.
Hardcore MUDs can be brutal and vicious because the stakes are very high. In some ways they are similar to MUSH atmospheres, except with no system of consent or safety; really, you are always at risk, which in some cases makes the victory sweeter. Often these places have the biggest IC/OOC separation, because people are busy playing the game instead of, well, not playing the game. Not much control over your character's personal story, but anything you can get, you've generally earned and will have to hold onto. Can be the hardest of all the settings to let your imagination wander far afield...in other settings, if you want a strange character idea, people either won't argue with you, or (MUSHes) they'll work all the kinks out in character generation and expect you to play it fairly with social pressure. In hardcore MUDs, expect people to be less credulous, and a concept of 'fair play' is much more important, so bending the rules for a certain character concept can be very hard.