The biggest thing GW has going against it is the misplaced expectations of players who already have ingrained in their heads what an mmorpg is "supposed to be". You can see examples of that here (ie thwart's post, no offense intended). Here WoW has the edge because it gives players what they expect, whereas Guild Wars is not easy to figure out and currently there is not enough in-game guidence.
As far as balance, I think it's pretty damn great. But that's not easy to figure out until you get a good group, and there is a shortage of competitive missions. For instance, it would be awesome to have a 4on4 where you can pick teams. Or dueling. Or two on two. 4on4 random or 8on8 with people you probably don't know is chaotic and favors everyone's favorite warrior/monks. BUT, I've been on teams of four where we didn't have a single warrior, and routinely kept decimating teams consisting of 3 warrior/monks and a monk.