Not really gonna happen any time soon.
WoW has figured out the perfect formula to tap into that previously untouched goldmine of anti-social, lazy people who dont know a damn thing about computers but every college kid practically owns a dell laptop it seems. They didnt go for flashing, intense graphics.. they went for ok visuals that look inviting and pleasing and best of all, will run on a PC that was top of the line for a month or two in 2001 and will work fine for people who still run windows 95.
Thats the thing... they made WoW for MMO geeks in the beginning.. then have consistently ratcheted it down over time to appeal to a literal mass market of bored human beings who wanted to justify their impulse purchases of low cost Dell computers and laptops. Then the economy hit a slump... people needed a way to socialize without wasting gas, spending money, or taking showers. Enter, WoW. With it's T for teen rating and bright colorful characters as opposed to the usual doom and gloom of the more serious sword and sorcery fare, it brought families into play.. it was a game 10 year old little timmy could play with his 9 year old sister and his 45 year old mom on friday nights while dad went raiding with the guys from Work. The same reason the Wii for all it's less than exciting game titles keeps selling like gangbusters. It's the appeal to people who like a little entertainment but dont like the idea of grinding through fantastically realistic piles of ogre guts. WoW is the chutes and ladders of the modern age, Nintendo is the Milton-Bradley.
Until someone else comes along with a game as polished as any major MMO on the market, but with the amount of accessibility of a Disney Characters on Ice show, WoW is going to dominate the market. Games like Age of Conan were cool, but seriously how many casual people had a PC capable of really playing it without the chop and lag? Not many. And it was NOT built for kids.
There will be no WoW killer until Disney creates Kingdom Hearts Online with Square Enix.