The lights are on
Coop are usually a option. But when your cornered by zombies, almost out of ammo, little health left, you better kick yourself for not following your buddies. Left 4 Dead 2 does this well, if you wander off from your buddies, your pretty much dead.
You play as one of four survivors, Rochelle, Coach Ellis, and Nick. Each area is pretty straight forward, for 4 levels get to from point a to point B, a safe house, and there's a finale where you have to hold up until rescue arrives. If you think you can just plow through these zombies, think again. The AI director can spawn special infected such as the hunter, the tank, the charger etc. If your health reaches 0 you will go down and if a partner doesn't get to you fast, your dead. Oh and don't wander away from your partners if you do your most likely going to die, making coop essential to getting through levels.
There are other modes besides campaign too. Verses is sort of like campaign but with player controlled special infected, the fact that some of your enemies are controlled by acuall players it make is even more tense. Scavange, where you need to fill up gas cans before time runs out, and survival which is pretty much your average survival mode
Overall left 4 dead 2 is a great addition to the left 4 dead franchise (the 2 games) and has great coop