By way of game review slash encouragement:
This game doesn't do a lot of new things, but it is executed *perfectly.* (in this sense it's the opposite of Dwarf Fortress.) The sound work is amazing, the graphics are cinematic, and the pacing and level design are flawless.
This is a game that rewards teamwork and severely punishes going it alone. What I love best about the game is the effortless multiplayer communication. Your in-game characters talk to eachother, and it's always relevant. Stuff like, "I'm reloading," or "hey guys I need to heal" or "there's a pipe bomb over here." Also, "why are you shooting me?" I don't have a headset microphone for my 360, and I find that with this game I don't miss it. The team notifications, and being able to see where your teammates are even when they're behind walls, means that a whole layer of basic communication is handled automatically, and well.
Aside from enjoying the genre, there's something about the co-op experience that I find really addictive. The only other co-op game I've played that hooked me this well was Natural Selection. I think what the two games have in common is a focus on co-op interface design; NS does it with the commander and hive sight, and L4D does it with frequent in-character voice cues. Both succeed because the tools allow you to create great little stories, over and over again, that the whole team participates in.
There's something about being a part of a well functioning team that we humans love (and the social tools we use can make or break the experience). Some people get this team experience from sports, or from a game like Counterstrike or what-have-you, but I think that what Valve have done with Left 4 Dead is lower the barrier to entry. Now you can have that team experience without having to have first mastered the game, and without having to ACTUALLY TALK TO ANYONE. That is impressive.