Words you will hear alot playing Left 4 Dead, perhaps the greatest co-op game ever built. The game oozes with quality, and is simply full of the trademark Valve-isms that Half-Life 2, and Team Fortress 2 have begun to make standards of gaming. From the thousands of lines of dialogue that may potentially be spoken, that dynamically re-acts to the action unfolding around you. (Although I've yet to find any dialogue as funny TF2's Spy being lit on fire "I seem to have spontaneously combusted.") Get your friends together, plug in your microphones if no one is in the LAN, and start killing off the zombie hordes.
Okay so perhaps you'd like a slight explanation of what Left 4 Dead actually is. Well it is a game that occurs in the zombie apocalypse. You and 3 other survivors find yourself trying to escape the hordes and well survive. The game does not try to be realistic, which works well. Pistols never run out of ammo, but do have a significant reload, and fairly significant damage-distance ratio making them fairly ineffective at long range. Survivors can take punishing brutality from zombies and never become infected. (The game explains by saying that you are the random element of humanity that is immune to the infections, at least I think it says that somewhere...)
Anyways, you start at point A, you need to get to point E. Which is your rescue. The levels are fairly linear, but have enough variety in the paths to make it possible for minor route changes to have significant impacts on gameplay. For example a shorter path may be known, but is guarded by a witch. You can take the longer route and therefore avoid her completely. Or you can just headshot her with the shotgun; but to be fair that is pretty damn hard. So along the way to Point E you stop at Safe houses which basically serve to slow the pace down, and also are when the new level loads.
The standard co-op is great fun, assuming you have somewhat decent teammates, but the game only truly comes to life when you play in VS mode where the boss zombies are played by real live human opponents. This mode is brutal. The normal mode gives you brief respites every after intense fights. But VS mode is constantly intense. New players spawn as the boss monsters. Although the Boss monsters always spawn in the same frequency. Two Hunters, fast leapers who fully immobilize their prey but must get to their target. One Smoker, long range tongue user who brings his prey to him. And One Boomer, who vomits on survivors which brings in the horde to assault the survivors, Oh and he can also go boom! when shot. Basically they will fuck you over. Badly. So badly that you will want to leave the game right away. It will be that bad.
But you should not. For after the hordes are done feasting upon your tender flesh you will get you hunt those who only moments ago slaughtered you. And you will make them feel the exact same as you felt.
Well, I should clarify, this is assuming your team knows what it is doing. If not, well it is not doing to be pretty. Although there are few things that are as redeeming as outsmarting a human opponent. Hell thats why we play multiplayer games. Beating real people just makes you feel superior.
So who should get Left 4 Dead? Um... well most people who should get it already have it. But if you like zombie movies, or enjoyed Valve's previous games but was unsure of this one. Well just get it. I'd recommend PC over the 360 version. But then I am very Biased towards the PC. (Or am I? I mean I dont exactly own any companies that directly benefit from the sale of PC games, and I'm not in the hardware industry. So I suppose I'm really not biased. Meh, go figure.)
So yea. Left 4 Dead is awesome. I hope you get it, and enjoy it.