A friend asked me tonight if something will eventually knock World of Warcraft off its perch as top dog and what such a game would look like. I do think eventually something will replace WoW as the 500 pound gorilla in its niche. And here I'm talking about the particular genre this game dominates; in terms of numbers alone, Farmville appears to clean WoW’s clock. But the games are so different in style, content, and business model, that it really doesn't make sense to compare them directly. And I suspect, in terms of raw income, but World of Warcraft still does better than Farmville. I'd love to see some numbers on this.
So what will the WoW killer look like? First, I do think Guild Wars 2 is barking up the right tree with their new classes. Giving everyone the ability to shift between tank, healer, crowd control, etc. is brilliant. It makes it a lot easier to put together a team to tackle any adventure. Once you've abandoned niche protection, you can then focus a lot more on what matters to the players. Play style is almost certainly the way to go. Making one class be about fast, precise clicking, and another about preset macros, and a third about positioning, and other such considerations makes a lot more sense.
Easing the grind is also key. WoW got rid of standing in line for your chance to kill the orc king. That's not fun for anybody. Replacing the grind with more interesting quests is probably the way to go here, and I think BioWare’s Old Republic MMOG will be the model in this regard. I haven't played that game either, but buzz says it has managed to capture a lot of the appeal of their single player games.
That said, don't expect story to be king. It would be extremely simple to give every player their own, unique plot and adventures. Nobody's bothered to do it yet, and that's because the players simply are not interested. Most approach quest text in this manner: "blah blah blah silver hammer. Blah blah blah five wolf pelts. Blah blah Darken Wood blah blah blah." Story is to MMOGs what those scantily clad girls serving drinks in casinos are to Vegas. They are an essential part of the window dressing, but nobody is under any illusions they are why the crowd has come. The folks most interested in story are quite happy with their free-form gaming; those who want to charge them $10 a month have nothing to interest them.
The graphics will continue to avoid rigorous realism, though I doubt the game that replaces WoW will look quite as cartoony. It will probably include neat cross-class combos like Dragon Age did. It will almost certainly be fantasy, and will absolutely be combat focused. It probably won't have much, if any, crafting system. It will be even more amusement park in its geography than even World of Warcraft. I suspect that no character over 10th level or so will ever really die; they will simply enter some form of bloodied or half-dead status in which they can escape to try again later.
Whatever it looks like, I seriously doubt I will be interested in playing it. It's been many years since I've been tempted to play a Diku MUD, graphical or otherwise. Most of the blame can be laid on the topics discussed in the story paragraph above. A game that wants to tackle those topics may attract my attention. I'm not holding my breath.