Prototype theory holds that we conceptualize through categories in which some members are more central than others. If I ask you to name a piece of furniture, you are quite likely to come back with “chair,” “table,” or “sofa”; if you immediately thought “armoire” or “ottoman,” you are weird; if you went with “Charles, or Susan if it’s a girl,” you are very weird. If you asked an American for the best example of a bird, the most bird-like bird around, you will get far more robins than penguins and almost no emus.
The usual concept of a western MMO seems clearly descended from DikuMUD, through EQ and terminating in WoW. I would tend to insert DAoC in there, sometimes described as “EQ without the parts that suck,” but I may be atypical. Perhaps I am uncreative, but I do not see much more room for the Diku model to evolve. It has reached its full flower in WoW. You can have refinements and variations (-raids, +PvP, +story, -classes, +Tolkien, -fantasy, +F2P), to say nothing of lousy clones, but it will take something massive to change the view of the most typical member. There is a lot of room (and money) in WoW’s orbit, but if you do not want to be (seen as) conceptually subordinate, you need to head a good distance away.
We have some less typical members, most notably EVE Online. You all know how I love to pull out “here is how City of Heroes solved that problem,” or how I mix a dozen niche games into my bloviations. These can be annoying in the MMO blogosphere when commenters contribute them independently, not in the sense of “here is an alternate way of implementing that” but rather “your entire argument is invalid because it does not apply to my game (or playstyle).” It is as if you were complaining about birds pooing on your car, only to have a passerby disdainfully remark that there are not any penguins in the area and they could not have flown over your car anyway. Well, no, that is not what I meant by “birds,” but thank you for contributing.
The archetypal example may not be displaced by a better example. I describe Torchlight as out-Diabloing Diablo (for the single-player experience), rather than referring to Diablo as an early example of a Torchlight-style game. (I think of Diablo II as the most typical member there; is that what most of us mean by “Diablo”?) Amusingly, the “most typical member” need not have ever existed. You probably picture some central tendency of a robin rather than a particular bird you once saw. You may think of Dracula as your most typical vampire, but while he never sparkled, he could walk in sunlight; see also Igor and brain-eating zombies. Because Halloween is coming.