For most first-person shooters, "co-op" isn't just an afterthought - it's a misnomer. Dropping a group of typical twitch FPS players into a standard single-player campaign and assuming they'll work together is like throwing a few balls of yarn into a basket of kittens and expecting a Persian rug. Instead, the end result is usually closer to parallel play than a genuine group effort; you only truly notice your teammates when they steal your kills and beat you to the power-ups.
If the game industry's approach to co-op changes in the next couple years, we'll look back at Left 4 Dead as the game that changed it. With the exception of those of the massively multiplayer variety, no game has been so singularly focused on getting players to work toward a common goal - in this case, survival.
In Left 4 Dead, you take control of one of four survivors of a mutated rabies epidemic that turns the Infected into a soulless (yet bizarrely heterogenous) horde of flesh-eating monsters. It's standard fare in the film industry, but for the glut of videogames featuring the living impaired in one form or another (mind-controlled Spaniards, frozen ghouls from the North Pole, undead curiously susceptible to properly spelled words and phrases), few games have managed to convey the salient point in zombie fiction: the need for people to band together to make it out alive.
Nearly every gameplay decision in Left 4 Dead has this objective in mind. Simply put, it's shockingly easy to die. If being swarmed on all sides by a pack of Infected and losing your ability to move isn't bad enough, there are three species of special Infected tailor-made to pull your group apart at the seams. Smokers are the snipers of the bunch; they lurk around corners waiting for a survivor to stray too far from the pack, then lash out with their prehensile tongue, slowly asphyxiating their victims as they pull them further from the group. Hunters are less subtle; crawling around walls and leaping from building to building, they're a bit like zombie Spider-men. If one manages to pin you, he'll claw at your face while you lay helplessly on the ground until one of your teammates knocks him off. Boomers are walking bags of zombie-attracting bile; they'll either vomit on you, distorting your vision and sending a fresh pack of Infected in your direction, or lumber up to your group and wait for a stray bullet to burst their over-inflated guts, drenching everyone in the stuff.
Then there are the boss Infected - tanks and witches. You'll know you're approaching one by the ominous background music and creepy, ambient sound cues. Tanks come tearing toward your group at full speed, swinging their hyper-muscular arms around like wrecking balls and hucking chunks of concrete at you and your allies. It takes a coordinated effort among all four Survivors to ensure that everyone gets away (relatively) unscathed. Witches, on the other hand, just want to weep in their corner and be left alone. (QQ more, IMHO.) Turn off your flashlights and quietly creep past them, and they're little more than scenery. Startle them and they come alive, flailing their claws and screaming like a banshee. One hit and you're on the ground; a couple more and you bleed out entirely.
As compelling as the Infected are, it's Left 4 Dead's much touted Director that steals the show. The Director is the behind-the-scenes A.I. that spawns enemies according to an arcane formula that takes into account your group's past performance, credit scores and various astrological data that are too numerous and complicated to list here. The net effect is that no two treks between safe houses are ever the same. It also forces players to constantly communicate about incoming enemies or risk stumbling upon the wrong zombie at the wrong moment.