Guild Wars is more of an online RPG, with the capability of small parties like Diablo. I haven't played an MMORPG since Everquest first came out, and have not been really tempted, due in part to the monthly fees in addition to full price software, and the focus on MMO. If you like to play single player or with small parties, GW is the way to go. If you always feel you need to be in a massively populated world, GW would not be the right choice.
I'm playing GW, and I have only 'partied' once for about five minutes to do one quest, and one more time in a forced, game-configured party when I 'seared'. I'm trying to get the missus into it, I'd rather party with her than a bunch of folks I don't know.