MMOs often get talked about like their subscribers don't consider single-player genres against their MMO purchasing decisions. The MMO genre might have a unique appeal to a lot of gamers right now, but that doesn't mean we've all got a mentality of "Ok, this game will satisfy my MMO craving, and this other game will satisfy my single-player craving." Many of us, when browsing at Walmart or Gamestop or wherever else, view both MMOs and single-player experiences in one category, as simply "games". If I find a single-player game that satisfies me, then I probably won't buy the MMO until I'm bored with that game.
MMOs and single-player games are in direct competition with one another. The essential difference in style is just one feature. There has to be much more to convince me the game's worth my money.
Alright, so maybe that's stating the obvious. Let's move on to my second point. These genres are merging.
There are already debates about whether the likes of Diablo 2 and Hellgate: London qualify as MMOs (I don't think they do). Defining an MMO's going to get a lot harder over the next 5 years. Interview after interview tells me that most developers (if not gamers, too) now consider online features a no-brainer for nearly any game. Honestly, I'm not the biggest fan of this trend, but it's happening regardless.
Many developers consider strictly single-player experiences a thing of the past. Sure, they'll still design single-player games, but those games must have a multiplayer mode or two... and the multiplayer is what is intended to keep the gamer playing for months. At the very least, the game must connect to a larger community asynchronously (like in Spore, where players affect each other without actually playing together). Why add replayability through game mechanics when the inclusion of other players can do it for you, right? And the number of players included in these games' multiplayer modes rises every year.
MMOs might be special now, but they won't be for long. A lot more games will fall into the grey area between single-player and MMO in the near future.