The problem that most co-op content faces is one of over-familiarity. There's something about a ten-minute long deathmatch that remains addicting and exciting, even a hundred games later.  Ploughing through the same content in a group, even a good group, quickly loses its shine. People typically don't want to play through a dungeon, like Karazhan, a hundred times; they're simply compelled to in order to gather the rewards. Replaying a single-player game doesn't have that, and most get bored of it quickly. The same goes for strategy campaigns. When you've finished the book, you don't want to read it every night. And since by that point, the AI's little if any problem, and you know the levels backwards, you may as well just turn the guns on each other and enjoy the far more satisfying challenge of a good scrap against an opponent who can play without cheating.

This doesn't mean that co-op is a failure. It's just a more limited experience, one which requires more of players and burns out faster. Played right, some of the best experiences on the PC are working with someone. However, without a stream of fresh content to keep the experience new and interesting, playing this way is more likely to be a one-shot experience than most multiplayer modes. It's no surprise that as people stop gathering around a TV to play games, the odd exception like the Rock Band series notwithstanding, the focus is on more team games and deathmatching. That's left co-op somewhat out in the cold, but with games like Red Alert 3 and Left 4 Dead to show everyone how much fun it can be, perhaps not for too much longer...