The idea is, to bring that same enjoyment to an MMO, to have every quest basically be as entertaining as any current single player experience on the market. It won't happen anytime soon, but that is the idea I'm talking about. How many single player games have you wished there were co-op in, so you could play through it with maybe 2-4+ other people in to enjoy the experience? If you haven't wanted that, I'm guessing that's a rarity, because I've yet to talk to a gamer that hasn't.
You enjoy the MMO aspect in what I'll refer to as a seperate medium, a universe of some sort where the mass of people reside, where you meet your RL friends, talk to other people, etc., and supposedly have plenty of general content to enjoy whenever you want in a fully-functional environment. When you want to be the badass (or in the small group of badasses, either one), you do your quest (this is kind of a bad word to use for what I'm thinking, but it's the most recognizable) and, technology allowing, it will be just as epic and realistically affecting as any single-player game you'd enjoy.
Until the point where MMO's can have this kind of experience, I think even the best single-player game could be improved by the availability of multiplayer, even on a smaller scale, perhaps involving just roughly 8 people or more. There are a shitload of possibilities with this in mind, but multiplayer will be the key in all of them.