First thing is first I am NOT saying GW2 will not be successful, I am saying it isn't, and can't dethrone WoW but can co-exist with wow as it's own new kind of game.
Why?
First of all the no subscription fee deal means it can't beat WoW's subscriber base, because it has no subscriber base. Which is a great thing for GW2 why?
It lets people play WoW and GW2 at the same time, since most people only want to pay 1 subscription fee at a time. Unlike swtor, or rift which have the compete with wow to be successful, Gw2 can coexist with WoW and be successful. IT gives it an MMO feel, yet a single player game price.
Completely different game models.
Don't get me wrong, WoW has a great model and it works, and I enjoy it, but when other games copy the WoW model I just feel like, 'meh I should go play WoW, same thing." GW2 is done completely differently, and doesn't really have hardcore raiding like WoW would, so it provides and off-time for WoW players to do when they are done with raids, or waiting for new raids to come.
So honestly I wouldn't be surprised if a lot of people play both Gw2 and WoW at the same time, like they might play LoL and WoW at the same time. Or WoW and Sc2 at the same time.
Guild Wars 2 won't take down WoW, but it can very easily be just as successful as WoW, and be king of it's own genre, while WoW reigns on it's genre. (F2P MMO genre and P2P MMO)