First of all, I have never played WoW. But from what I understand, there are some pretty major differences.
1) GW level cap is at 20. The game does not encourage level grinding, instead it concentrates more on skill-based playing. If you want to grind, you can do so to get titles.
2) Skills. You'll reach lvl 20 pretty fast in the game and after that you need to rely on your skills. You have slots for only 8 skills so a little strategy goes a long way to beat those hard bosses.
3) Gameplay is instanced. This means that the world map is not free-for-all. You can take player groups of about 4-8 people (depending on where you are in the game) and once you leave town/outpost with these people, it's going to be just your group against the environment until you reach another town/outpost. While outside a town/outpost you will not bump into any other players and you can't kick anyone out of your group.
4) If you want to try PvP, you can create a character especially for that. This means your PvP character will immediately be lvl 20 and have access to best armor and max damage weapons. You won't be able to play PvE with that character though.
5) From what I understand, GW has a more intensive storyline. Each chapter has a clear storyline that you can follow at your own pace and which brings you into new areas.
Um... and I think you can maybe find a bit more about GW in this thread:
http://www.guildwarsguru.com/forum/s...php?t=10102502
It helps people who are trying to decide which campaign to buy.