Co-op gaming is older than most gamers; while examination of co-op gaming has always existed (Co-Optimus.com, for instance, is an entire site dedicated to co-op gaming), many recent releases have prompted players everywhere to think about the state of co-op and its capabilities.
In fact, one might aruge that Epic's Gears of War 2 is the culmination of what co-op gaming always was, while Valve's Left 4 Dead and its unique game mechanics are truly a step toward the future.
Gears of War 2 followed a formula, and did it very well. It is a shooter complete with content, a campaign, and various multiplayer modes. One of these modes, horde mode, brings the players together in co-operation, a first for a title like Gears of War.
In Gears of War 2, co-op is doing what it has always done: adding even more value and longetivity to the game, creating incentive not only for consumers to buy the game, but to keep it; also, the more players that keep the game, the fewer that have the option to buy used. From the side of production, there was always the incentive to provide co-op modes so that more people would purchase the game.
Turtle Rock and Valve, on the other hand, saw that co-op could have other uses; instead of making co-op an added feature, why not make significantly different design decisions based on co-op principles?