Played both WoW(for a year and a half) and Gw(coming up to 2 years in 1 week) extensively,played them both on and off...needless to say I quit WoW 1 year ago and havn't gone back. GW is an acquired taste. Its very laid back and theres no need to have to schedule your life around it which is nice. The expansions are as said above 3 different games,but you connect em all to once account and your characters can hop through them. The TRUE expansion to the game is coming out at the end of this August. WoW and GW are completely different games, uncomparable infact. GW is unique and has a heavy PvP base and a large PvE base and a mix of both too. There is interaction, though its very limited to outposts since its an instanced game. You don't really worry about it too much when you find a good community(alliance) to play with. You can try and get a trial key or i may find one laying around that I can give you!
Forgot to mention, the guildwars team is constantly putting events every weekend for you and whatnot as well as keeping up with updates and adding random crap to the site...imo they are alot more active than the WoW team or any other game team in terms of providing random events and updates.
More about the game...theres 3 seperate stories and theres many different things ya gotta do. Similar to WoW, they have "elite missions" which are missions you enter with a group that are more challenging than the ordinary. You get nice items there and all that good stuff. The skills are many and theres a unique part to the game where you can make your character have its primary class(for example, a warrior) and then a secondary class(lets say an elementalist, which is kinda like a mage). With those kind of combos,you can use the skills of both classes and make some interesting set ups. The game calls for strategic minds as it is not like other games. It only allows you to have 8 skills at a time(hence you have to strategically configure it for whatever the purpose may be). Many find it annoying, but they just lack brains i suppose. Just don't buy the game until you try it!Its an acquired taste as I said and rushing into it might leave you dissapointed.
If you are going to get one...i'd say get either Prophecies(original) or Nightfall because the first has the story that the upcoming expansion will be continuing, and the 3rd is way better than the 2nd(factions). Either of the 3 will let you play the 4th one. If you can though, try and get all 3 of them because it'll really help when you have access to all the skills in the game and the different armors(each game having their own armors and whatnot).
@DCIScouts: I totally agree with the whole diablo resembalance...though 3 of them worked for Blizzard aswell as D2 and Battle.net so i can see why
HOLY SMOKES THIS IS FROM 2 MONTHS AGO ALMOST!
I just typed so much for a post that means nothing to the poster anymore