Relmstein has posted a quick analysis of what he calls the "punctuated equilibrium" of WoW content patches. In evolutionary biology, there's a theory that species change not gradually over time, but in quick bursts of dynamic change. And Relmstein applies this idea to WoW's own population changes-- the playerbase seems to grow in quick leaps when brand new content is introduced, but slows down and even falls off when standard bugs are being fixed, or not much content is being patched.
What's really interesting, however, is that Relmstein then compares WoW's changes to the effects that content schedule has on other MMO releases. Lord of the Rings Online and Guild Wars (which are WoW's two worthy opponents) both released during downtime (after Burning Crusade and after the vanilla release, respectively). And on the other side of the spectrum, both Vanguard and Everquest 2 tried to go directly up against new WoW content, and, as Relmstein says, got steamrolled.
So looking towards the future, it's not hard to see what might happen. Wrath of the Lich King will make a big splash for sure, both bringing lots of players back, and maybe even bringing new players (who played Warcraft III and want to see Arthas) into the fold. Games like Age of Conan and Warhammer Online may try to go up against it, but it wouldn't be a good idea-- they'd be better off waiting until about a month after the expansion, when many players have reached 80, seen what they can see in Northrend, and Blizzard is confined to bugfixes and small content updates. Of course, a WoW content break isn't all these games need-- they still need to be good games by themselves. But placing themselves in this downtime between new content will give them a much better chance to woo more players away from Azeroth.