A big problem with many MMOs is that once you finish off an area there is little or no insentive to return. As a consequence, those maps turn into wastelands which is a huge turnoff for new players that log in and feel like the game is empty. GW2 has done a good job of trying to find ways to encourage people to return to old maps, and keep the sense that the game is healthy and alive. Plus, there is so much to do and explore in this game, I just like going back, even if its just to wander around and admire the beautiful design of the game.