Generally, the MMO with the biggest "wow" factor for a gamer is usually the first one they got invested in long term. And its likely they're actually just being wowed from experiencing the concept of an MMO rather than the actual game. The beginning of the Warcraft thread here is pretty telling of that - "huge world, we're all playing together, amazing!" etc.
Many people still claim Ultima Online was the best "wow" factor MMO ever because that happened to be the first MMO they got heavily involved in. I'll bet that in 5 years time, for many people, GW2 will have been their first MMO and nothing will match the "wow" factor they nostalgically associate with it.
Either way, whether a seasoned MMO player or new to the genre, GW2 is definitely doing something new. Its somewhat iterative rather than truly revolutionary but it gets the most important part spot on - its a helluvalotta fun!