L4D is pure win!
Graphics- The graphics aren't the greatest but certainly not the worst. It may not have as impressive graphics as say Gears of War 2 but they are still pretty good especially the character/infected models. They are so detailed.The only real complaint I have is that your arms don't look to real but that is only minor.
Gameplay- The gameplay is easy to learn and should only take about a level or so to memorize it completly. It's not to simple yet not to complicated.
Story- Not much of a story here. You basically know the story without even playing the game. There is a zombie apocalypse and you play as 1 of 4 survivors attempting to survive and escape. There really isn't any stroy what so ever. Well technically there is but you know what I mean.
Replayability- This all depends on whether you have Xbox Live or not. If you do you will be addicted to this and be playing for countless hours in front of the TV. If you don't however it might not last you quite as long in single player but it is still a very fun game to play and you will come back for more eventually.
Fun Factor- This is possibly the most entertaining and fun game I've ever played. From times where a Boomer vomits on you and the Horde comes charging to attack you to extremely threatening Tank attacks and everything in between. You will not get bored with this game guaranteed.
Overall- Overall L4D is a great game that you really need to check out and purchase for sure. It is far to addictive and fun online or offline to only rent it for a few nights. You need to buy this game ASAP.