4 people with guns against 1000 mindless zombies....hmmm...what are the odds?
Left 4 Dead puts you in the role of 4 survivors thus the punt Left 4 Dead...and you can choose to play as 1 of them... which really doesn't make any difference.
Fighting hordes of zombies and shooting them is the main part of the game... really like a brainless shoot-em up... but to make up for the mindlessness shooting (because you are not the zombie most of the time)...you are task with protecting the the 3 survivors which could either be bots ,people you meet online or friends.If 1 dies...it's probably fine...if 2 dies...you have a problem...once 3 are gone...you are screwed.
So other than shooting zombies after zombies... you essentially have to ensure that your pals are able to survive as well otherwise it'd be impossible to make it through the whole level.
The downside of this game is the limitation of campaigns or maps.. giving you only 4 to choose from... and the 4 actually feels really similar after a while...
But with friends playing along with you and screaming at each other for help...you will soon forgot about the repetitiveness of the map and start screaming along with your friends.