I've been playing games for 25 years and this may be the best experience I've ever had!
Graphics: I'm not surprised to see negative comments about the graphics; the game uses the source engine which is now four years old. At times, objects and textures can look unrefined and pixelated. However, there is good reason for this: there is so much happening in the game, so many zombies on the screen at once, that this is necessary. Visually, a game like Gears of War 2 is better looking. But Gears never has as many enemies and action on screen that Left 4 Dead does. This has to be taken in consideration when reviewing the quality of the graphics.
In fact, I think the game looks great. It's a forest vs. tress situation: when you are stationary and examining things (which is foolish when there are flesh eating monsters afoot), you may find unimpressive textures, but when playing, you see just how brilliant the graphics look and how well the game runs. There is no frame rate slow down! The game has such a great feel with the graphics because of great pains the developers took to use special effects to get that "horror" look. The animation is great and things seem very lifelike despite so many undead walking around! 9.5/10
Sound: The sound of the game is right on. When there's little to no action, the only sound you will hear are groans from zombies or movements from you and your teammates. However, when a boss or hoard rushes you, the music, in true horror film fashion, becomes frantic to capture the desperate situation you're in. Often times, the survivors (main characters), will make comments appropriate to the situation, such as "healing", or "smoker", to indicate a boss is near. If they are injured or hurting they will let you know. Guns are right on. 9.5/10
Gameplay: This is undeniably the best co-op multiplayer out there and where the game shines. The quality of this game, as has been said, truly depends on who you bring in with you to help fight the zombies. If you're a loner in the world of video games you best change your ways and make friends fast. After all, if this event happened in real life, would you team up with people who refused to listen and cooperate?
What is so amazing in this design is that it captures not only the element of teamwork, but the art of communication. Players who can effectively communicate and listen to others will survive this game; players who don't will perish. How simple and yet brilliant! Your very survival in this game requires the constant cooperation of teammates, something games like Army of Two and Gears come close to, but don't quite capture this the way Left 4 Dead does.
However, the highlight of the gameplay is the AI director. For those of you who don't know, the AI director randomly changes the sequences of the game every time you play. One play through a boss might be in one setting but in other play through he doesn't appear at all. In other words, you never know what to expect and this adds to a unique experience each play through that greatly extends the chance that you will be playing this month to come.
Addressing Criticism:
My one complaint, albeit small, is the lack of story. While most horror films are hardly noteworthy for their stories, it would be nice to have some dimension to the characters you play as.
Other criticism that you may hear would be about the single player. First of all, this game was designed to be multiplayer from the start. In essence, faulting it for its single player experience would be like faulting counter-strike for its single player experience- it makes little sense to do so.
You may hear the game is too short with only 4 campaigns. Alas, I wish there were more campaigns, only because the game is so great. The game has 4 campaigns with 5 levels each, so all in all there are 20 levels. But consider this: playing the first campaign on the expert difficulty, I spent at least 4 hours on it and didn't beat it. If you play the game on easy you may be able to run through it in 2 hours. It's really up to you.
In conclusion, this game is a must own, and my vote for game of the year. It's really up to you, the player, to determine how much you get out of this game. If you buy it for single player, you will be disappointed because it's not built for that. If you don't make an effort to make friends with people who are committed to communicating, you will be disappointed. You get what you give, and if you give a lot, you may just survive to the end with a smile on your face and your friends by your side!