I just wanted to write my piece in this thread because ...as you can see from my avatar, I love the Left4Dead experiences. This is simply why.
Left4Dead takes you and up to 3 of your friends into a world where you have to survive together. The brilliance of L4D comes from how the zombies react to you and your mates. You're left with just your shot guns, some grenades, a health pack and your wits. At its prime, this game was being played by people all the time and the legends of a survival mode was still a dream that was months away when I remember this experience. My buddy and I were playing through a map for the six millionth time. A tank showed up and threw a car at another random player. He jumped in the air and got hit. My buddy ran after him as I tried to keep the tank on me. I watched as it died and jumped up and down in happy glee. I saw my buddy helping the other guy up. I then saw a hunter jumping towards him. I yelled into my mic "Harbenger301 no!!!!" as he was pounced. He dropped teh other guy and like a mad action film I ran over to get them.
This is how the game become so awesome. If anyone is reading this and deciding whether or not to get this game. Yes, do it. But do it with the understanding that you might want to just get L4D 2 because playing with other people and those experiences that follow is what makes this game so amazing and awesome and sets it apart.