When you break Left 4 Dead down to its core components it's a relatively basic game; you grab some items, run through a level shooting countless common infected, fight a few special infected, and eventually seek shelter at the next safe house ad nauseam. Left 4 Dead has a bare bones selection of weapons and items at your disposal, so playing it by yourself gets very old, very quickly. Fortunately Left 4 Dead has an exceptionally well-realized cast of playable characters and a fantastic online co-op experience that help keep it interesting from start to finish.
Left 4 Dead deliberately skimps on the story and begins two weeks after the initial infection that has turned all but a select few into ravaging brain-dead zombies. In Left 4 Dead you'll have access to four playable characters, all complete with their own vastly different personas: Bill, a hardened Vietnam War veteran; Francis, a tattoo-covered biker; Louis, a Junior Systems Analyst at his company's IT department; and Zoey, a college student and horror film enthusiast. This is as far into each character's background as you're ever going to get, but it's their dialogue, personalities, and the way in which they interact with one another that keeps them interesting.
As one of these four characters (and hopefully with a group of friends) you'll trek through various locations chock full of infected just waiting to ravage you and your comrades. You begin each level armed with nothing but a pistol, but you'll have access to either a shotgun or an Uzi, plenty of ammo, and some medkits. Along the way you'll come across various items scattered across the levels, which includes pain pills, Molotov cocktails, pipe bombs, as well as superior firepower. Unfortunately Left 4 Dead's weapon selection is painfully limited and you will only have access to five main weapons in the entire game. Eventually you will be able to upgrade your Uzi or shotgun to an m4 or an automatic shotgun, both of which serve as upgrades to each of the starting weapons, as well as a hunting rifle which can be used as a sniper. Left 4 Dead's gunplay is kind of weak though. It's not really bad but the lack of weapon variety makes it pretty underwhelming in general and the weaponry is not nearly as satisfying as that found in Valve's other games such as Half-Life, Counter-Strike, and Day of Defeat. Oddly enough there are no melee weapons to speak of in Left 4 Dead which seems like a glaring omission, especially considering you're in the zombie apocalypse and all. You do have access to melee attacks with your firearms but they serve more as a shove than an actual melee attack and do minimal damage at best.
Left 4 Dead's level design is pretty good though, and the four main campaigns have enough variety to keep the gameplay experience interesting. Perhaps what gives Left 4 Dead the majority of its longevity is the AI director which alters the gameplay experience depending on how the players are performing. Unfortunately the AI director isn't really as significant as Valve would have you believe, as weapons and items generally always appear in the same few locations. The infected are generally everywhere so it's hard to really take notice of how differently they may have been dispersed this time, especially since the levels always play out the exact same way despite the minimal differences.
You will also encounter various types of special infected throughout the game: the hunter, smoker, boomer, tank, and the witch. The hunter stalks players from various angles and can leap great distances to incapacitate and maul the survivors. The smoker can snatch players with his long tongue from great distances and drag them off to their doom, he'll also explode into a cloud of smoke when defeated, obscuring vision. The boomer can puke on players blinding them, as well as alerting their presence to a horde of zombies which will then hone in on their location. The tank is a rare special infected who usually spawns during climactic moments, such as a chapter finale. He can easily incapacitate an entire team of survivors if not quickly dealt with, and can take an incredible amount of punishment. Finally you have the witch, who will not attack players if left undisturbed, but when provoked can incapacitate a survivor in one attack. The special infected are what keep the gameplay interesting, as their appearances are much more infrequent and unpredictable, and generally require the team to work together to bring down. The special infected can also be controlled during versus matches in multiplayer (with the exception of the witch) which can lead to some amusing moments.
Playing Left 4 Dead co-op definitely gives the player the best experience possible, and you will have to rely on your teammates to make it through all of Left 4 Dead's campaigns. You will constantly be attacked by hordes of infected, and the special infected's unique abilities with often incapacitate players to a helpless state where they will need to be saved by a fellow survivor. When players are incapacitated you will have to revive them, and you'll earn points for doing so. You'll also earn points for protecting other players by shooting infected that are just about to attack them. Friendly fire is always enabled and cannot be turned off, so you'll have to be careful not to shoot your friends. You can heal each other with med packs, and you can give pain pills to each other with temporarily restore health. When you're incapacitated after a serious injury you'll have to wait for a teammate to revive you, and in these cases you can only use your pistols. This can lead to some intense moments as the rest of your team struggles to survive as you blast the infected swarming you with your pistols from the ground.
The finales featured at the end of each of the campaigns hold some of the best moments of Left 4 Dead, as you and your friends protect a fixed position from a seemingly endless horde of infected. If anyone is left behind as you eventually make your way to the escape vehicle the game will display 'In Memory of' listing the players name, along with all of your gameplay statistics listed as if they were movie credits at the end. It's fun to compare your stats to those of your friends, and the way it's all presented makes it a lot of fun.
Left 4 Dead's presentation is perhaps one of its strongest selling points. Each level is presented by a poster showing all of the characters (as well as the names of the people controlling them) topped off with a cheesy catch phrase, reminiscent of campy horror movies. The sound design is excellent, and easily one of the best aspects of the game in general. The music is appropriately foreboding and contains repetitive loops that get stuck in your head and immerse you in the zombie apocalypse atmosphere. The zombies themselves actually all have sound effects provided by experimental musician Mike Patton, who actually made an album based solely around him making weird sounds with his mouth. So it's not really very surprising to see him voicing zombies in a video game, and he does an excellent job. The voice acting is also superb and breathes life into all of the characters as they talk, scream, and laugh together. It really makes them feel like actual people struggling through the zombie apocalypse rather than a bunch of mindless drones void of any type of personality.
Left 4 Dead's replay value is essentially defined by how high a tolerance you have for repetition. There are currently five campaigns in total including the recently released Crash Course DLC, but the campaigns themselves are fairly short and all play out in an entirely similar fashion. With a significant lack of weapons and overall gameplay variety you're probably going to wear out on Left 4 Dead pretty quickly. So unless you have a high penchant for doing the exact same thing over and over again you're probably only going to play through Left 4 Dead once or twice before calling it a day. The versus mode can provide some additional amusement after you've gone through the campaign normally, but it's also riddled with annoying players who hog items, use cheap or otherwise irritating tactics, and people who like to play lone wolf style rather than playing as a team. You're definitely going to want to be playing with people you know if you try any of these modes. If Left 4 Dead had more content it would be an easier game to recommend, but as it stands it's a relatively short experience that is significantly lacking in long term replay value and it's sole long lasting benefit is it's charming characters, excellent audio, and entertaining but brief co-op play.