Honestly, the physics engine of WoW. Or whatever it is that makes the abilities and combat "feel" more impactful. Might be just a really good combination of animations and sound effects.
Not sure. But there is something about GW2 that makes me feel like when I'm fighting or running I'm not actually interacting with the ground or my opponent. Not sure if that counts, and it's sort of abstract, but it's really the only thing that I think this game lacks.
Also maybe some of WoW's streamlining, every time I take a break and come back to GW2 I feel more and more lost and confused.