Well I played WoW for around 10 months 2 months after release. And after getting to the end-game, after a while it was such a bore I couldn't stand it, and I got GW a bit after I quite.
I'll try to give you a comparison type thing.
Game:
First off, GW isn't a one world type game, everything is instanced. Exept for towns and outposts.
The Good:
No competing for spawns, no ganking, no harassing by other players, etc.
The Bad:
Not near as much player interaction, you can't just walk by and see people farming or questing and such. Or people can't come and give you a hand mid-way though a zone, you would have to leave pick them up and restart.
PvE:
In GW is quite different that WoW, like most things in GW it's based on your skills, and the skills your character has. Also, you can only have 8 skills on your bar at once, where in WoW you could have anything and everything on your bar at once.
The Good:
Like I said before, it's much more skill based, so a lot of the time it's not as much how high a level you are, or what armor you have, it's how well you can play. Personally this is what I like about GW the most. Also, classes have singular roles in WoW, in GW your class can do whatever you want, within reason of corse. Here is a good example.
http://guildwars.gameamp.com/guide/viewGuides just take a look though all the guides, and you can see for your self. Some are good some are bad and some just suck, but the thing is you can make....going out on a limb here and saying millions of builds, with hundreds (thousands even?) of skills.
Also there is no huge raids or anything of the sort, I happen to really like that, as raids in WoW where just bore-you-to-death grinds. The biggest group you will be in for PvE is 8 and I believe elite mission is factions is 12? Also the end-game type zones, Underworld and Fissure of Woe, are a challenge for the most part. I happen to like to 2 man, but to explain that I would have to type a lot, and I believe there is a good explanation n this site about "55"ing
The Bad:
You never get the "I am a god" kinda feeling you got on WoW when you have a full raid geared character, but I don't miss it that much. It's easier, not that, that is such a bad thing, but I've always like things a bit difficult, for a challenge ya know. Where in GW it's kinda easy, at least for the first part of the game. Factions is a bit harder, but not by a lot.
PvP:
A lot of the same thing applies here as with the PvE, much more skill bases etc. As well as it's all instanced, in factions you can fight for control of certain areas with if you win enough. There is a lot more on that on the GW main site I'm sure.
The Good:
Like before, skill based! No level 60 "Uber" tier 14 raid equipped guy gona come up and ass-rape you while you are farming/questing, or in battlegrounds. Kinda odd coming from someone who was one of them for a time, haha. Even though I was never one to gank lower levels unless they made me. Anyway, it all revolves around your abilities and how you use them.
The Bad:
No world PvP of any sort, as the entire game is instanced. But I suppose you could look at it, as it ALL is world PvP as the world is instanced. :P But then from what I have heard (from friends) world PvP doesn't happen much in WoW either even though it has been some time since BGs where released.
I hope that helps, if you have any more questions just ask, I may or my not see them though. :P
