Its two completely different types of MMO games, one is designed from a PvP perspective with some added PvE, the other is designed like most others, out of a PvE perspective with some PvP.
GW2 has no raids, a decent amount of dungeons with different types of gameplay options (story/explore). It has tons of PvP, balanced and fun, both for those who enjoy BGs or RvR (world PvP for those that never played DaoC).
Questing is nothing like in WoW, no even similar, same goes for skills, quickbars, combat mechanics and traits (talents). When it comes to graphics and envoirment, maps and all that, WoW feels very flat, old and boring compared to GW2. You cant walk up on a mountain in WoW and say "damn what a view" and actually see a keep in the distance you can visit.
I liked WoW very much for 7 years, but I wont be going back, I've been off WoW for a year or so, and it makes me happy. WoW was a drug, very mainstream and easy to keep interest in. GW2 has brought a whole new feel, well not new, nostalgia from games predating WoW, awesome games.
I sat down yesterday and did some WvW, it was the most fun I've had since raiding the frontier in DaoC, real MMO PvP battles with a purpose and not some WG/TB piece of !@#$ proxxy.