I see a lot of MoP is awesome posts in here and so far it is, but just like Cata started off as fantastic xpac, so is MoP. We don't know if Blizzard will continue the Xpac off well, or if this one will end up just like the last. Maybe they do need a couple years of work do produce 3-4 months of good content.
As for GW2, I have not played it yet, though I have an account. I did play in some of the betas and it was good, though not nearly immersive enough for me, not like the first day I played WoW, where I had a whole world around me with so much to do. GW2 doesn't feel like that, though it brings some good concepts to the gaming market. From the responses I've heard on the game, it's really hit or miss, you either love it, or it was just an attempt to dethrone WoW.
As far as the next MMO, I don't think that will come without new technology, something like the Occulus Rift, where you might actually feel like the world is around you and not just in front of you, that's what will kill WoW and bring the player base to a new game, because right now, although WoW has fallen from it's graces a good bit, it is still the best MMO out there.