It is the multiplayer that really shines.
The only original thing about this game is it's title. Valve uses a quote "Left for dead" and alters it to give the meaning of four people left to die. The game's setting is very basic: Four survivors, immune to a psychosis virus, must fight and defend themselves from the infected in order to survive. The game offers several modes, including a story mode. Here, the four survivors must fight their way from one safehouse to another in order to find safety. That is basically your main objective for each level.
L4D is a very simple game, but it is so well made. The game offers an awesome variety of weapons against the zombies which include sniper rifles, shotguns, molotov cocktails etc. The levels are loads of fun to play in and there are several different types of the infected. Especially if you are playing multiplayer and you have swarms of zombies charging at you, then things get extremely intense.
The graphics are nice, but the gun modeling looks very cartoonish and reminds me a bit too much of Team Fortress 2. Despite that, the beautiful lighting that the Source Engine always features, combined with detailed environments and very few slowdowns makes the visuals look satisfying enough. The sound of the guns and zombies also enhances the mood if everything goes silent and you are forced to be alert for ambushes.
This game is not as innovative or deep like other Valve games such as Portal or Half-Life, but it is one of the most addictive games Valve has released. The single player leaves a lot to be desired since team member AI are not very smart. Regardless, L4D is a solid buy and if you love fast-paced shooters with zombies that are meant for multiplayer, I guarentee you will have lots of fun.