lol WoW Killer.
Well I could write for England on this topic, but I'll just jot down the general gist of my thoughts. I don't think WoW was a particularly exceptional game on release, in that there were several other MMOs around the same time that were of a similar quality (and since then I'd say there have been several superior titles). There was an element of luck in WoW becomming the dominant player in the market. And it wasn't intended either; Blizzard wanted a place in the market, sure, but it wasn't a realistic goal (or probably even considered possible at the time) for one game to be so successful.
Since then, with Blizzard being Blizzard, they have added a lot of polish that cements the game as quite a solid title, and the amount of content available is obviously far beyond what any new game could ever be released with. But more than that, its place as the market leader is what makes it unbeatable. MMOs are about community, and the fact that WoW has the most players reinforces its position against any competition. In effect, I don't think it's actually possible for a new game to be a WoW Killer while WoW holds such a dominating position. WoW will die out eventually (and it could be many years yet) of its own accord, of course, but it won't be another game that does it. I don't think even a WoW2 or WoS would stop people playing WoW.
On the other hand, the big factor of Guild Wars 2 is that it doesn't have a subscription. Don't push that aside for a single moment; that is huge. When you're considering two subscription MMOs (say WoW and SW:TOR) then for the most part you play one or the other. Most people won't pay two subscriptions at once. But many WoW players will still play Guild Wars 2. That's a very significant factor.
No, Guild Wars 2 will not kill WoW. It should hopefully develop a niche group of players and have a good set of content to play through, but it won't be competing in the same market. There is no way that Guild Wars 2 will ever be as successful as WoW. In fact, I don't think there will ever again be a game as successful as WoW. One thing that is slowly dying for sure is subscription gaming. If Guild Wars 2 is a success then I doubt many future MMOs will be made with a subscription model.
Mildly relevant aside: Jeremy. Soule. Fucking hell, what a god.