Best of all, you will have different stories to tell from each game because they will play out differently every time. The Director AI makes this possible.
This is a special AI (and an evil one at that) that controls everything that happens you to in the game based on how your team is doing and how you play, both individually and as a team. It controls the music (which will often be different for each player), where the Infected will be, the type of Infected you will face, how many there will be and when they will attack.
It also decides where weapons, med-packs and other items will be in the map. So just because a Tank attacked you by the petrol station the last time you played doesn't mean it will attack you there the next time you play that level or that those ammo reserves will be in the same place.
The only moments that are kind of the same are what they call "Crescendo" moments, which are times where you usually have to activate something which will bring a horde of infected your way (a great example is when you have to wait for an elevator to get to your floor and fight off A LOT of Infected while waiting). There are also the finales at the end of each level. With these you will have some time to prepare before triggering the event, and then you have to survive for ten to fifteen minutes while a lot of Infected will come at you until your rescue arrives. This makes each play through unique, fun, scary and extremely challenging. It's also the reason the game has great replay value even with only four movies.
I should mention that all four movies are completely separate to one another, and that's a shame because I feel it was a missed opportunity to tell a more cohesive and interesting story. Instead, you have four completely separate stories about the same four characters and they just seem to have absolutely nothing to do with each other. It would have been more interesting to know how the characters go from one predicament into the next. On the up side, this also means that Valve can easily release more movies as downloadable content and don't have to worry about making a story fit with it.
I really do hope they do this soon because right now it's tough to argue that there is enough content. With only four movies you can get through the entire game in about five to six hours. Yes, there is an enormous amount of replayability in it and you will still have tons of fun playing through the same levels over and over, but it definitely would be nice to have more options, especially in the versus mode which is only available for two of the movies.
So speaking of versus mode; I honestly was having such a blast playing co-op through the single player story as the survivors I thought the versus mode was going to be kind of a tack-on extra. I was wrong.
Playing as an Infected is even more fun than playing as the human survivors (but that's probably because I can sometimes be a jerk who likes to screw with people... Tehe).
Versus mode involves a total of eight players on two different teams. The one team plays as the four survivors, and the other as either a Boomers, Smoker or Hunters (occasionally one player will randomly get to be the Tank). You have no control over what you spawn as, the Director decides after every time you die. The survivors have quite a lot of health and the infected, not so much, but you will respawn (often) after ten to thirty seconds depending on the situation. The point of the infected is to stop the survivor players from getting to the safe house in that chapter. It takes a bit of getting used to and some may not like it at first. However, once you get your head around it, it is so much fun!
As soon as the survivors are either dead, or some of them get to safety, you all switch sides and it's then your turn to survive. Points are then given to each team based on how well they did as survivors. In a lot of games the survivors will never get to the safe room, so the way to win is to just make sure you get further than the other team did on their turn. It's extremely challenging and so much fun! It's very satisfying (not to mention funny as hell) as the Infected team try to take a team of healthy survivors all thinking they are safe, and just destroying them in a matter of seconds. For the survivors team, versus mode is even scarier than the regular game because the boss infected are coming at you more often than usual and you never really get to feel you're safe.
If there are any complaints to be made about Left 4 Dead, they are really just a matter of quantity. The quality of what you get is great (it looks great, plays great and sounds great) but only four movies really does feel very limited. While the PC version is almost guaranteed to get plenty of free extra levels, the Xbox 360 version might not be as lucky. Firstly, unless Valve is able to make some deal with Microsoft it's doubtful any additional content will be free. Secondly, who knows when any of this new content will be out? The PC version has no issue here, because Valve have always been great with providing regular new content, plus the modding community have already been busy at work making this game playable in the maps of other Valve games; a luxury 360 players won't get to enjoy.
Still on the topic of quantity, some may feel there are too few weapons in the game. I will admit the pickings are pretty slim and a little more variety may have been nice, but still, what is there works well and fits to different peoples style of play.
Before I wrap up, there is one control design I feel is worth mentioning regarding the Xbox 360 version. If you tap the right bumper button your character will spin around 180 degrees. It's a small feature that allows you to quickly turn to look behind yourself. At first it was awkward to use and I barely used it but as I played more and got used to it I realised it's actually really genius. I just hope more console FPS's begin to use a similar feature. It really goes a long way of bringing to the Xbox 360 controller a similar speed as the always-superior keyboard and mouse controls on the PC. So, with that out of the way; I'm going to stop wasting time writing this review (I hope you're all happy) so I can get back to playing...
If you don't intend on playing Left 4 Dead with friends or people online, then you aren't getting the right experience. You will still enjoy it and have fun, but this game is meant to be played with other people. It is the best co-op experience I have had all year (which is saying something, because Gears of War 2's Horde mode is also fantastic). Much like Counter-Strike, it is incredibly addictive but far more random and atmospheric.
If you enjoy FPS's and playing with friends, Left 4 Dead should be very high on your Christmas List this year. With all the great games that have come out in the last 3 months, this is the game that I will still be playing into the New Years. No doubt about it.