Some pondering on Left 4 Dead.
At first glance one would not hesitate to throw a 5 star rating at Left 4 Dead. It's jsut so damn original and clever.
The game focuses on the adventure of 4 survivers as they traverse through 4 scenarios, or "movies." Each movie has a differnt feel and setting and will generally last between 45 mins to an hour.
The main selling point of Left 4 Dead is the co-op system. The way in which these 4 players must intereact with one another in order to achieve success is pivitol, especially on the higher difficulty settings. Use of the microphone is vitol in plan creation, and therefore vitol in achieving success.
The AI director does a great job of mixing things up, by setting particuly nasty zombies loose on you when you are doing well, and always making sure that items are not in the same locations twice. Thus placing a small importance on exploration in an otherwise linear shooter.
The only downfall of Left 4 Dead is the play time. 4 Chapters is just not enough, even with the AI director mixing things up ever so slightly.
Best played with friends.