This always interests me because of ArenaNet's roots in Blizzard's Diablo.
I find it ironic that the presence of GW has helped WoW improve *its* PvP. I think it's pretty obvious that the honor system and the new Battegrounds PvP competition update to WoW was put in as a direct method to stop/slow down attrition to GW.
And it works, at least in my guild experience. 3/4 of my guild has left GW and went back to playing WoW. Other long-standing guilds that I know (from Shadowbane) have had a similar time attrition going on. Granted, my guild had played WoW a lot before GW and many of them picked up GW to try it out. I am told the Battlegrounds has a more FPS time feel to it that PvPers enjoy, with capture the flag games etc. but all using their WoW characters that they've developed over a few months. Who knows how long it will last, but there were some things I noticed that I didn't expect beforehand from talking to these guys who have gone back to WoW.
1) The lack of a monthly fee in GW encourages people to try GW, but the presence of a monthly fee in WoW encourages people to play WoW more if they have both games. It makes sense, actually, because if you are paying a monthly fee, you feel like you ought to use your WoW account. Whereas with GW you have no qualms about putting it on the shelf for months at a time.
2) I am neither for nor against a UAS, but I think the lack of it has hurt GW's long term marketability. It won't show in initial sales, because the consequences of a lack of UAS don't show up until you're about 50+ hours in the game. Some people have claimed that GW and WoW have different crowds and compete very little with each other, but I think that's wrong. There are a lot of people who were disappointed with WoW who picked up GW but did not cancel their WoW accounts (who would, after putting a few months of work). For these people, GW has to offer a quick and promising gaming experience before WoW gets altered to try to suck them back into WoW. I think the UAS would have helped here, as it would have quickly showed these straddlers what GW has to offer that WoW cannot patch- depth in organized PvP that goes beyond skill acquisition and levelling.
There's also the sector of the gaming population that WoW can never attract: the player who simply cannot put in the time commitment (I'll be PC and avoid the charged word "grind") to compete in WoW, as well as the FPS/Counterstrike crowd who like to log in for an hour or two after work, kick some butt, and that's it because of time constraints. GW, rightfully or wrongfully, advertised itself to cater to this crowd, but to the extent that it provides a barrier to entry as far as "work" that must be done in order to "get" to competitive PvP, it is not attractive for someone who could just log on to CS or similar games. Again, I am not saying that GW should have a UAS, but if it doesn't, it definitely loses its appeal for *this* crowd.
3) This has been much commented on, but the lack of a monthly payment means younger players. Parents can just buy the game for their kids for the summer and keep them happy. Is this good or bad? I'm not bashing kids, but I think it's fair to say that a large proportion of extremely young players making a large portion of the online gaming community is not conducive to realizing the organized community PvP potential that GW was designed for. My experience has been that I've played with significantly more 17 and under players in GW than in WoW, Shadowbane, UO and even many strategy games. There is a certain glee that occurs for young players when play their first online game- the freedom of anonymity to curse, be an ass, etc. (ok, I'll admit it, I was probably like that too when I was 16 years ago

I'm not saying there are solutions to any of these issues, if they are even problems. And, even though there's no monthly fee, it does affect every GW lover if the community becomes small or fractured- it means less expansion purchases and less good stuff in the future. Arenanet has to maintain their servers after all, and they will only invest in expansions if it provides a marginal return for them.