World of Warcraft, Mabinogi, Hello Kitty Online -- these are all titles that make a fantastical graphics aesthetic work well for them. At the same time, Age of Conan, Guild Wars, Tabula Rasa -- these games make a good showing of a more "realistic" style, and people seem to like it.
Then again, there's "realistic" taken to an extreme; we're thinking of the way The Sims makes you exercise, eat, sleep, excrete, etc. Now, while The Sims is of course not an MMO, do we want that sort of thing in our fantasy worlds? Does realism attract more people than surrealism? Could you take currently realistic/cartoony games and switch the aesthetic around and make it all better?