First of all, there's no monthly subscription.
As for game play:
The biggest thing for me is how events and quests work. There is the system called pandemic(i may have the name of them wrong) events, where you can walk into a city and watch it get attacked. While watching this happen you will receive a quest to defend the city. Also, this is happening to everyone. You won't save the city just for someone else to save it afterward, it will stayed saved(or destroyed depending on if you succeeded) for other people. They want to get rid of "go kill ten cows" quests.
The second biggest is there is no holy trinity(Tank,Healer,DPS). Whatever class you choose to play, you'll be very effective and able to solo the game. That's another cool thing. You don't need to play in large groups or even with anyone at all if that's how you like playing.
In my opinion, game play is the sort of thing you need to feel for yourself. (IE: Lots of people love WoW, but when I tried it I didn't think it felt very smooth to me.) These are a few features that will rock.
A few pluses:
They have wonderful painterly textures if graphics are your thing. Also, Guild Wars, in my opinion, has the nicest and most helpful community. You find a lot of other games have mean, no nubs allowed, attitudes. Although we don't bash on players who aren't as good, i'm not saying GW players aren't very good. We are plenty skilled, just nice.
Here's a link to a GW trial: http://www.guildwars.com/freetrial/