Ah, I remember WoW's gripping quality well. The need to constantly gear grind just so that my character could be relevant for the next bit of content.. Personally putting in as much time as possible into the game to "Get the most" out of my sub fee. e.e (That's what gripped me anyway, I can't speak for others.)
Mainly for GW2 though, what keeps me interested (beyond the frequent updates) is just how massive the world is, honestly. I've played since the closed beta, and participated in the head start, yet there's still little bits in the world that I've never explored or discovered yet. Also, the community is relatively great over-all. Of course there will always be assholes, but I've personally found GW2's community to be one of the nicest I've experienced in my MMO history. And finally, the lore, I looove lore in games, and the whole Scarlet debacle aside, I just love GW2's story. (Yes, this includes GW1's)
I take breaks every now and then, go and play other games sometimes. Helps to keep you from getting burned out. But for now, anyway, it's the one game that I keep coming back too. Much like how I played WoW, only now I have more freedom with playing other games!