This term has been used now for... a few years id say, Every new mmo was label'd this way and they all flunked, then theres Rift, I think rift has done what games like AoC and WAR didnt do, by offering a game that has appeal to the avarage WoW player, i like to think im not biased, i played wow for along time, and like some i got tired of it, 5 years of any game will burn you out eventually, ive played other games since stopping wow after cata, but have now settled on rift, Why? becuase its not a complete style change from wow, it has elements of wow but through similarity you will gain subscribers.
AoC really was a let down, i bought it and think i played it 3-4 times before just giving up completely, trying to introduce somthing such as a new combat style can break a game and i think AoC tried to do everything unrelated to wow in the hopes people would simply convert to a new style.
So, my thoughts on a "WoW killer" honestly, i dont think there will ever be one, as years come and go new games will come out, blizzard will lose its subscribers to wow eventually, but they bring new games out just like any other company, so why you might not play wow anymore you might play Sc2, or D3.
If there ever was going to be a wow killer, by that i mean take the majority of wows playerbase id put my money on there new MMO, which technically wouldnt be a wow killer as the same company will generate the same revenue if not more.
i wish people wouldnt use the term wow killer, as there never will be one, they lost 600,000 subscribers lately, sounds massive and it is, but when you compare that to the 11.4 MILLION they still have, you cant honestly think 1 game will take the majority of them, which btw, would mean at this present time 1 single new game would need to gain atleast 5.7 million subscribers, and wow would have to lose the same number. Love or hate the game you cant argue with numbers.