I'm happily playing GW2 right now and it's accessable, reasonably challenging and fun. And the fact that it doesn't require a subscription commitment makes it pretty painless for WoWaholics to try. It's made by guys who really know how to design games - who are knowledgeable about how it's done elsewhere but who don't mindlessly mimic successful formulae - and it shows in a lot of small and big ways.
I am a bit concerned about its staying power, though.
-You essentially get all of your abilities - save for some very long cooldowns - within the first few hours. There's a lot of abilities (25ish that you can have equipped, 75ish for your class total), but the "shape" and general strategy of your character won't change over time like it does in WoW.
-Areas feel smaller than WoW's. Part of this is how much more convenient everything is - you're just running around filling nodes instead of doing questlines, so there's less of a sense of direction and place.
-There's no mana bar or equivalent. Abilities cost nothing to use. They have cooldowns - some of which are very significant - but taking the resource-management dimension out of the game makes it a bit flatter. I don't have the feeling of wanting to hunker over a torn-out piece of paper, theorycrafting my character.
I'm not sure that the GW2 guys are even *aiming* to dethrone WoW. Their business model seems not to be the subscription MMO's call of "Stay awhile. Stay FOREVER!" but instead "Here's a bunch of content for you. Once you're done, grats! Get lost. Come back when we have more ready."