My favorite game of all time.

I have never played a game as good as Left 4 Dead. The team aspect is really rare in the world of Video games. Left 4 Dead has successfully taken out the mentality of running around guns blazing and killing everything in site. You have to stay together, and be careful what you shoot or you WILL lose. It is also really good for replay value. I can STILL play any of the original four "movies" and enjoy it as much as I did the first time. A lot of people aren't happy with the lack of storyline, but if it had any more story in it online play would be a pain, and replay value would be significantly less. Left 4 Dead is not an rpg, it has all the storyline it needs.
Also, most people who play Left4Dead are friendly, and fun to play with. These are the only kinds of people that could do well in such a team based game.
Just my opinion, but Left4Dead is without a doubt, awesome.

Unfortunately the more recent updates, have ruined it a little though, the new map is crap. The lobby system is now broken, since the game puts people into game before it tries to fill up a lobby. So it can be hard to fill up a lobby. Even so, it is still really fun to play, and was worth every penny spent. (plus some)