There are no subscription fees, which means once you buy the game it's yours, most likely meaning they won't add in patched content except to fix bugs unless you buy it because it would be pointless to give things to players who aren't paying for them. (For example in WoW you pay every month, so although patches are free, they keep you playing and paying), while I know some gamers will literally play whatever game, single player or multi-player over a long time that was never even intended for the game, but most gamers will play the game and once they beat it they move to something else. Although in other Pay to Play MMO's like WoW people will keep playing the game because the content keeps coming and it keeps giving them stuff to do.
I get they can do expansions, but wouldn't it have been better for GW2 to make it pay to play and add patched content, because otherwise I do not see how GW2 will survive with a constant player base for over a year. It seems like a great game, but does not look profitable for them (not that I should care besides the fact that making profit is what keeps them making games).