"Attaching laser sights to your weapon makes it more accurate." Yeah right. Actually, I have no idea whether attaching laser sights to your weapon in Left 4 Dead 2 makes it more accurate. It's hard to verify, even after you've shot 7500 zombies during your first day playing it. But attaching laser sights to your weapon certainly makes it look cool.
The laser sights aren't the best thing in Left 4 Dead 2. When you first encounter them halfway through the opening campaign, Dead Center, they're not even the best thing in that room - the man who wants you to go across the road and get him some Cola is. But they are symbolic of a broader change to a game that looked to some like a cash-in: whereas Left 4 Dead was about getting people to co-operate, and a nervous Valve was reluctant to complicate things in case people misunderstood, Left 4 Dead 2 is about making the most of the fact people did get it, by having some fun with the concept. Because people really, really did get it.
On the off chance you didn't play the original, Left 4 Dead 2 is a first-person shooter designed for four players to play together. You pick from one of four characters, choose a campaign and then work with one another to survive the zombie apocalypse the game unloads between the starting point and the last chopper out of town.