Well, what they quickly found out was that AoC isn't a very well done game. WoW really is a slick game. Not perfect by any stretch of the imagination, but polished and quite a bit of fun. This is why they have so many players.
While I don't play WoW right now, I have played a bit in the past (but only to the mid-thirties). I will agree with you wholeheartedly that it is not a perfect game (again, only made it to the mid-thirties), but that it is probably the single slickest game made that is also on such a grand scale.
I didn't realize this, however, until I played on some free servers in an attempt to accelerate my leveling just to see what some of the game's later areas were like. Maybe I was playing on a particularly bad "fast leveling" server, but I could tell that such an interruption to the game's delicate balance really wrecked everything.
I'd be at level one, with zero experience accumulated, and it'd be awesome to kill that first wolf or boar (or whatever woodland critter that I'd be fucking frightened to see in my backyard) and shoot up eight levels and collect sixty gold.
However, this immediately nullifies the usefulness of all quests in the area, so you're stuck with traveling already. And then as soon as you get to enemies who will shell out experience, you realize that all of your attacks are missing and you're getting pounded, because you skipped eight levels' worth of weapon/defense proficiency growth. Since you nullified the usefulness of all of your earlier quests, you're stuck grinding. Immediately.
Basically, I'm just trying to say that WoW's slickness comes from the developers' strict attention to balance -- even the player economy in WoW is a pretty beautiful thing.
I've been trying to find another MMORPG to play so that I don't have to be another "WoW junkie," but I don't know how successful I'll be. I haven't tried AoC, and after RTFAs, I won't. I'm currently playing Lord of the Rings Online, but I can't help but think that it's nothing more than WoW wrapped in Middle-Earth. And the player base is vastly smaller, so finding people to group with can be a chore.
I'm eagerly awaiting the release of Guild Wars 2, however, because the original Guild Wars is such a phenomenal game...once you get past the fact that games with no monthly fee attract a lot of idiots, and idiots don't handle the character customization that GW gives you very well...