Seems commonplace these days to always have WoW fearing every other mmo on the market. since every other mmo failed in taking down WoW, its left to question what remains of the pool. The answer is this, that pool then has to try and retain a larger portion of those interested in mmo's who may or may not be entrenched in WoW.
Theres a few mmos either out, coming out which i think Arenanet should be very fearful of.
1) The Secret World: just went B2P (buy once, play forever). Love the story, combat and the modern setting. Has a fairly worthy following
2) Star Wars The Old Republic: Recently went free to play, seems to have picked up a continual increase in attention since then
3) The Elder Scrolls Online: If any mmo coming out in the near future its this one. Will do everything GW2 claimed to do, but better and will also satisfy the WoW crowd.
4) ArcheAge: The next true sandbox mmo (albeit themepark partially) Build houses, ships, create your own faction, create a truly customisable class and conquer other players lands.
thats a start. Since GW2 was truly heralded as the next 'big thing' i think Arenanet have a LOT of work on their hands to keep up. If you have any further mmos to add to the list feel free.