Gamasutra recently compared the co-op goodness of Gears of War 2 and Left 4 Dead. Michael Walbridge, the article's author, argues that Left 4 Dead "represents bigger changes" for co-operative gaming.
Most co-op to this point has simply been meeting the need for players to play together while not playing competitively. The players could thus experience the content together. Left 4 Dead does not have traditional content or competition (if Valve achievements are any measure), yet sold well and is still highly-played. Why?Feel free to read more about how the co-op in Gears of War 2 is almost formulaic when compared to the co-op found in Left 4 Dead.
It could almost be called an MMO in a box; you are out in the hostile world, monsters anywhere, and you don't know who you will have to engage these monsters with. Left 4 Dead can only be conquered if the players have the level of cooperation that a dungeon full of huge ogres and dragons requires.