For the veritable cavalcade of new titles in development to make it in the lucrative massively multiplayer genre, they're all going to have to face the challenge of standing up to the "holy trinity" that is World of Warcraft, Everquest 2, and EVE Online, at least according to Lauren Michele on her blog Mystic Worlds. While there is certainly room for niche titles to make their mark (a category she sets aside for the likes of CoX and Guild Wars) every new game in the MMO space, whether they like it or not, has to compete with the big three. Casual-friendly games have to compete with the polish and spit-shine of WoW, more intense games, both in terms of graphics and mechanics/lore have to compete with EQ2, and any game with aspirations in the PvP or economic niches have to go toe to toe with EVE.
While I might not be inclined to agree with the specifics of her characterization, I think the larger point Lauren makes is extremely salient to developers working within the genre. While second-tier first-person shooters get compared to Halo in the same way that new MMOs get compared to WoW, those new shooters aren't typically competing for those same dollars. The same can't be said for MMOs. While it's true that MMOs are one of the few growth spheres left in gaming, a fact which is largely responsible for the rather dense gaming horizon ahead of us, those new games aren't just going to have to compete with one another -- there's a whole litany of old titles that continue to hold players and represent direct competition. It's about to get a lot tougher out there.