Lets not forget that most other MMORPG's charge for major expansions too. How many expansions has EverQuest put out? Sure, you do get some content additions for your monthly fee, but they don't tend to be all that major, and GuildWars can make those same types of changes for free, very quickly, and easily with their auto update/patching system.
I've played GW in the Free For All events, and on a couple of the beta weekends, and while I find that it holds promise, I'm not sure it's quite there yet. I'd like to see more focus on the Player vs. Environment areas and less on the PvP stuff, although I know there are tons of people who feel the exact opposite of me. I like the concept, but I'm not quite sold on GW yet.
WoW on the other hand is a full on, no holds barred MMORPG with all that comes along with that. Monthly fee, tons of players online at the same time, plenty of quests, and a live team dedicated to improving the game experience. But make no mistake, there will be expansions, and you will have to pay for 'em.