I agree with a lot of winkio shared. I also agree with Blizz and NAMK in that it depends on the type of audience that the developers want to meet. Take my tastes, for example. Even though I usually stick to single player games, I've found that I do really enjoy the COOPERATIVE aspect on online games. I like the feeling of having someone else's back and them having yours, and the sense that the group would fall apart without any one of you as you face more and more difficult challenges and grow stronger together in order to face mightier foes and all of that kind of thing.
However, I strongly dislike the COMPETITIVE aspect of online games (or anything else, really; I know that life tends to be a competition, but that doesn't mean I have to like it
.) I almost always downright refuse to do anything that is directly player-versus-player, and I sigh at the inevitable comparisons made even in cooperative groups - someone always has to be "the best."
A few other random bits: I love a sense of progression; this is probably one reason I like RPGs so much. It's one of those funny things that's kind of hard to explain. Take, for example, level scaling. I hate level scaling. I despite it with a passion. I even dislike it when you're killing dragons for 30,000 experience, and you run across a kobold, and it doesn't give you the 4 experience that it gave you back when 4 experience meant something, though this is something that I can let slide as long as I can still laugh at how easy my lowly kobold foes have become.
From a strict game-play perspective, level scaling SHOULD make the game more fun - it means that you don't have to wade through useless, time-wasting encounters, and it means that you have to use actual strategy even in the simplest of fights against enemies from a long time ago. But it doesn't, for me, at least. I've never liked it. Again, I'm not really quite sure why I don't like it, but to me, it kind of breaks the magic of the game. Of course, level scaling in an MMO is a bit of a different beast on its own, but I'm starting to get off on a tangent here.
On a slightly similar note, I like open world exploration, but it still needs to have direction. I could never really get into Minecraft for that very reason. Grand Theft Auto, in addition to just being the type of game that I wouldn't find interesting whatever the case, suffered from those same problems when I tried it - there are a million and one things to do, but there's not really any point to doing them.
All progress in games is an illusion anyway, since you're just manipulating bytes on a computer somewhere while staring at a screen, but on some level, there is that imaginary sense of progress, and a sense that your character is really developing through your actions, and to me, that's what makes it fun.
I actually think WoW is a pretty decent game. It's far from perfect, and it's far from being the best game out there, but it does have a lot of good aspects that sometimes I feel are discounted on account of "LOL it's WoW." Yes, it suffers from the majority of issues that winkio mentioned (and is the epitome of some of them, such as the whole race to the finish idea. And don't get me started on the toxic community....) However, as I think I mentioned somewhere around here before at some point, if you play it with a group of people you know and with your own agenda, it can be a blast, despite its flaws.
TL;DR / Conclusion: Riding off the ideas that many of the rest of you have mentioned, my ideal MMO would have characters that need to work together in order to accomplish goals - and not just through numbers (eg, challenges that need three people to beat simply to arbitrarily require three people,) but through cooperation of synergizing skills and abilities, coupled, to some extent, with player skill. These characters, however, also shouldn't just fit into cookie cutter builds, (ie: classes are either a "healer," "damage dealer," or whatever, and it doesn't matter who you have in your group as long as they fit the necessary cookie-cutter role,) but should allow players to customize their character is such a way that they don't ever feel exactly the same as someone else, especially someone else with a largely different skillset. Somehow, these groups would go out to explore the world and have adventures, gaining virtual wealth and power, while at the same time, making it feel like a joint effort, and not a competition to be wealthier or more powerful than others. This is where a lot of winkio's ideas about an AI driven "story" of sorts would come into play. I know that what I have in mind is probably not exactly the same as what he or anyone else has in mind, but I figured I'd share.
That was way more than I meant to write; this is something that I always find interesting to talk about, though, so I figured I'd throw out my two cents. I don't blame you if you just skimmed the whole thing. XD