Hopefully, roleplayers won't feel too insulted by this post.
Kill Ten Rats has an article up asking why there isn't more role-playing in Dungeons and Dragons Online? We can extend the question to ask why--since MMOs stem from pen-and-paper (PnP) roleplaying games like Dungeons & Dragons--relatively few people roleplay in MMOs?
I'm going to say that it is because roleplaying in an MMO is fundamentally different than roleplaying in a PnP game. And the difference is significant enough that the majority playerbase sees MMO roleplaying as mere affectation, irrelevant window dressing that kind of misses the point of the underlying game.
The thing is that the point of pen-and-paper roleplaying is "conflict resolution in character". You play a character, you are presented with conflicts, and you resolve them in character. It's sometimes hard to see this in D&D because so much of the rulesbase concerns itself with combat. But if you look at indie PnP games such as Dogs in the Vineyard, where conflict resolution is more abstract, it becomes really obvious.
But in an MMO, you can be in-character all you want, but you cannot resolve conflicts in character. You are limited to the options provided to you and the need to share the same world with other players. You can try and spin "extra" conflicts between other players, but those do not have same weight as the conflicts the game itself provides. It's not "roleplaying" per se, it's just amateur theatrics.
(As normal, we pause to insert the standard EvE Online disclaimer. This is mostly because EvE Online gives players the tools to resolve said conflicts: ship-to-ship missiles.)
So I think that role-playing is not really relevant to MMOs, and isn't really something that should be expected from the players, regardless of the lineage of the genre. If players want to indulge in RP, there's nothing wrong with that, but I don't think it is something that game developers need to spend time worrying about. And I don't think that the presence or absence of a roleplaying community has any bearing whatsoever on the quality of an MMO.