From Gauntlet to Left 4 Dead - the history of co-op gaming

We'll use World of Warcraft as our example game, because it's the best and if you think otherwise, we'll burn you with mage fire (mages being, of course, the best class, especially Alliance ones).

Simply playing in a group isn't the cooperative experience people get hooked on. In those fights, you're typically just lending your damage dealing/monster distracting/player healing skills to a simple fight.