I really like the set up GW2 uses and the story lines in an MMO are fantastic. Sure single player games have a better story lines but for MMO's I really think GW2 nailed it. I would like to see some changes in Wv3 and I like how Anet has already started bringing in changes to the PvE side. I am excited to see the changes in lost shores or what ever it's called.
Bottom line on longevity in my opinion is the younger gamers will not stick around but as some one already said I think the days of only playing one MMO are gone for that croud. I only play GW2 right now and being F2P is great. But a lot of my younger friends play 2-3 MMO's at a time. I will not make the MMO ADD comments I have heard in the past because I feel that is crap. I just think that the hardcore gamers can blast through content so much faster than it can be created. When you try to make a game for casuals AND hardcore gamers you will either leave people in the dust by putting out content to fast or lose the hardcore players because it isn't fast enough.
So in short I think GW2 will be just fine. It's free to play so when you don't want to play it you are not paying for a subscription when you have blasted through all the content. The more casual gamer like myself can take our time through the game and get into all aspects of what is given to us. The hardcore players will come back when new content is added and as the changes to Spvp and Wv3 are made.
I personally and the rest of my guild feel we will be playing GW2 for a long time to come. But again we are casual gamers. When I say casual I do not mean we don't play often. We pretty much play 5 out of 7 days a week. We just don't plow through content. We finish every zone and we don't join the zone zerg to accomplish completion, we do it in single groups. We are also into crafting and a lot of the other aspects of the game. For me personally doing everything in the game is what is keeps me coming back. When I am bored with one thing I move on to another and back as my mood at the time changes. I think the "instant gratification" of todays games and gamers has changed the way MMO's function. Back in DAoC if you wanted action you made it, in todays games you click a hot join button and bam your in the action. I see why this is appealing but I think it is contradictory of what an MMO is. That is for a whole other post though
Sorry for the wall of text










