I dunno how valid this is now that everyone has broadband, but I remember back in the day that co-op was difficult to pull off because of internet lag. When 2-4 people are in the same room on the same console, there's no lag, even if there are 100 enemies battling it out with your party of friends.
But take all that data online, spread the players across the country or world.. and it's another issue, perhaps. Or it may have been a while ago. World of Warcraft and other MMOs can get away with "massively multiplayer" stuff more easily, I'd imagine, due to the semi-turn based nature of the game.
Even today, I think co-op is hindered because video gaming is, for most people, a pick-up-and-play ordeal. They don't call their friends up and say "Hey, let's meet up online and play." They just play with random strangers. And strangers usually don't make good co-op buddies. They'd rather just frag each other or work as a team against another team, but not really slog through a big campaign with just the two of them.
Anyway, I think a big thing co-op games need to do is to emulate the "watching movies or TV with friends" experience. People do these things and discuss the story together. But for most games, people play together as they would a sport.. and not so much as they would a movie. To really expand things, we need the movie-going experience too, I think. There needs to be more stuff that sparks real conversations between people, which movies can do. Well that's my two cents!
Adventures of a video game mercenaryhttp://virtualmerc.blogspot.com