An RPG should never require more levelling than happens during normal play (i.e. you go into a dungeon, you come out on the other end, you have enough experience, at least if you didn't run from every fight). RPGs are mostly played for their story, not for their combat (and traditional RPGs don't involve a lot of combat unless you want to play Hack&Slay) but since the lack of a GM reduces interactivity computer RPGs tend to require a bit more combat and less non-combat. Japanese "RPG"s are not really RPGs because you don't play a role, you play the combat sequences and watch the cutscenes.
Sure, the players need to stay in an MMO to create a social world but unfortunately they're doing it by the wrong means. An RPG should NEVER be a race to the finish "look ma I got level 60", then it's not a ROLE PLAYING game. But the userbase for MMOs seems to consist of a lot of people who think you have to "finish" the game by reaching the max level, getting all the "phat loot" and the other players are only people you have to compete with for that loot.
Of course a race to the finish-style game will be hard to keep fun all the time. That's why games like that tend to last maybe twenty hours offline. And the severe difference between a high level and a low level character makes for a lot of rushing in these games. What about a game where you have maybe nine levels to get that give you new tactics and abilities but don't make you some invincible killing machine? I think most cyberpunk (or generally futuristic) RPGs are pretty close to that idea, a bullet to the head will kill even a veteran.
Games like Guild Wars tend to do pretty well despite a very short levelling process by introducing much more player interaction (grouping in every instance, PvP missions, etc). And the hardcore will stay long after they've hit maximum level because they feel they have a function in the world. Be a famous blacksmith, member of a guild of assassins, king, whatever. Those who have a social environment they feel happy in will stay no matter how long it took to reach maximum level and get all the 1337 gear.