Left 4 Dead is a solid, gory zombie hack and slash game, with some great action and rather amazing visuals!
The answer comes in several parts, but the key to it all is an artificial intelligence routine with the imposing name of the Director. The reference there should be obvious, given the games B movie roots. At all times the AI is looking over your shoulder, evaluating performance and tailoring the game accordingly. It makes decisions such as when and where basic zombie hordes will spawn, how the dynamic music will rise or fall to create atmosphere, and where weapon caches and special items will drop. Between the Director and routines that control both zombies and your fellow survivors, Left 4 Dead is a triumph of AI. As the Director keeps things moving and changing from game to game, the enemy AI does a superb job of turning each zombie from a rotting flesh bag into a credible, frightening threat, while the friendly AI ensures that the other survivors in your party always have a helping hand ready, should you need it. And you will. The AI has action often, as it fills out the other three survivor roles. The skillful play you'll see, which is often more useful, if not more entertaining, than having other human players on your team, makes a joke of the cooperative AI in games like Gears of War 2. The CPU survivors will shoot distant threats, swat away that Hunter zombie that just pinned you to the ground, and generally have your back at all times. They occasionally get hung up in little environmental corners, but in 20 plus hours of gameplay I've yet to see any quirk that breaks a level or even acts as more than a momentary hang up. Without the excellent zombie designs, the Director wouldn't have much to work with. Tall, stooped Smokers attack from a distance with their tongues, which constrict Survivors and pull them away from the group. Hunters violently pounce, gigantic Tanks require real group unity to take down, and corpulent, waddling Boomers vomit bile to attract the nameless sprinting zombie horde. Witches add a weird stealth element to the gameplay, Skirt by the crying Witch and you'll be fine, but disturb her and the guilty party might be torn to shreds. In every case Valve has superbly animated the characters, giving them weight and personality. Wildly creepy voice work give voice to these reanimated corpses, and the individual sounds of each special zombie are recognizable enough that you'll be on your guard long before you actually see a Boomer or Hunter. This is the evolution of the arcade design document. Lack of narrative, static core characters and limited enemies collude with one intent: to test your skills and endurance. These special Infected do more than add variety to the Horde. They reinforce the need for players to stick together. A lone player attacked by a Hunter or Smoker is toast, once in the zombie's clutches, there's no way out without help from a friend. I've never played a game that needs so much work from a real life partner or an AI controlled partner, but they both deliver fun and enjoyment! The social mechanics that come to life in Left 4 Dead aren't quite morality and aren't quite fascism, but somewhere in between. How many games create a scenario where three people will wildly fight to save a fourth, only to give up when they collectively realize that the cause is lost? And while players sacrificing themselves to divert enemies are hardly unheard of in gaming, there's a palpable sense of loss in some missions! The game's design elegance shines through in your interaction with the controls. Everything is simplified to the point where an Xbox 360 controller works perfectly. Complex controls aren't needed to switch weapons or heal teammates. Yet the basic control set is a gateway to complex situations. As the Director throws unexpected things your way, and as hordes of zombies, normal and special, prepare to tear you and the other Survivors to pieces, there's never the feeling that you're also fighting the controls to survive.
I've gone this far without mentioning the game's masterful use of light and shadow, and how Valve keeps the maps dense and intricate, but eases navigation with well placed light sources. I also didn't detail the "crescendo moments", in which player groups are given a momentary breather before a massive zombie assault begins, or the simple but balanced weapon selection, and how it can work to a well-trained group's advantage. More than anything else, I appreciate the strategic qualities of the team play. Where Gears of War 2 is satisfying in a brutal, reactive way, Left 4 Dead invites more planning and cooperation. The Horde mode of Gears 2 is a good start for sheer against the masses gameplay. But the unwitting team of the Director and other players intent on grinding the scrappy Survivor group into hamburger brings co op play to an entirely different, and far more satisfying level!
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Good Points: Great AI, good visuals, amazing story and interesting characters, good mission objectives, awesome box art, cool lighting effects and some amazing gameplay ellements!
Bad Points: Boring story, a few bugs and glitches, some annoying sounds, rather "dead" last level.