2. The fact that you have to team up to survive. While the single player campaign pretty much makes you the point guy for each mission, you still have three computer-controlled teammates who are there to help. You can't go charging off on your own, because a hunter zombie will pin you or you'll get caught up in a smoker's scary-ass tentacle, or you'll just be overwhelmed by a crowd and won't be able to push 'em off as quickly as possible. Consequently, you're rewarded for saving your teammates from similar fates, reviving them when they've fallen, healing them up when they're hurting, and they'll do the same for you. Your combined firepower (and combined first aid) is your key to making it through alive. I really like that.
3. The survivor protagonists. Such great film archetypes! A grizzled veteran who heads a group of troops once more into combat -- he's too old for this shit. You know the type. There's a mild-mannered IT professional who hasta learn how to use firearms, and quick. A psychotic tattooed biker who's frankly having the time of his life running around shooting everything he sees. And a horror movie aficionado who prides herself in having the knowledge necessary to survive thanks to watching so many films. It's the last character who, by the way, calls "zombie bullshit" on the fact that the undead they're facing aren't shambling all Romero-style, but running like mad a la 28 Days Later. Seriously. If you haven't heard that snippet of random dialogue yet, you'll love it when she starts in on it. I know Simon Pegg of Shaun of the Dead fame gave a raspberry to Valve for creating fast-moving zombies, but at least the devs acknowledge this a bit.
4. The fact that we don't know the backstory. How'd this undead virus come about? Why's it spreading? Why are some folks immune? Who's behind it? Hell if we know, and we're not supposed to. The closest we get are clues in the saferooms between levels, graffiti from people taking the same routes to evacuation points. Even then, what we read are rumors ("I hear there's cellphone reception still in Riverside"), observations on the undead, and some poignant messages left behind for friends and family ("Sam -- waited as long as we could for you, but we have to move on. I'm so sorry!" One of the messages is from a kid who says he and his mom and dad are okay.) It's such a little thing, but it adds such a human dimension to the game. Valve learned, I'm guessing, that the graffiti in Portal really struck a chord with gamers.
5. Along with this, there's the fact that your goal as a survivor in this game is to, well, survive. You have to get from your hopeless position to some kind of evacuation point. Someone says they're evacing people at a safe spot nearby. Hooray! You fight your way there. But it's already overrun by the time you get there. Now what? Find safe harbor. This is all you do. This is your goal. Escape. Somewhere in this game universe there may be a Gordon Freeman character running around silently working with the scientists and government people to find a cure or distribute the cure or just kill the head zombie behind the whole thing, but that's not us. We're just the average Joes in the game universe, those lucky few who haven't died yet and are looking to keep it that way. I really love the fact that I'm not the One True Hero here. I haven't been chosen to Save The World, and I'm not going to. I just need to live long enough to hop on that helicopter when it finally arrives.
6. And the final map of each campaign is brilliant. This is another key part of a good zombie rush film: the survivors holing up in some stronghold and facing a never-ending stream of undead rushing at them, breaking through doors and busting through windows, all trying to get at them. Can you hold out until the rescue vehicle arrives? The odds are indeed not in your favor, and that's just a fact of the game. Just like in any good flick, someone's gonna die all martyr-style before the rescue. One of the characters you've formed an attachment to. It might even be you. In any other circumstance, a death so close to the end would feel arbitrary, but that's just how it is when them zombies come for you. Nobody's safe. You'll always remember good ol' Bill, who did his damndest to hold off the hordes while the rest of you ran for the chopper, but was eventually overwhelmed and killed as you took off for a safe haven. Such sacrifice on his part, giving up his life to give you those precious few seconds of time.
It happened to me the first time I made a dash for the rescue vehicle. I had provided cover fire for my buddies when the chopper arrived, and ran up to the helipad to make my escape. Suddenly there was a gigantic mutated undead freak (what the game calls a tank) in between me and the chopper. He'd jumped up just before I'd made it. He swung at me and knocked me off the platform, right into a giant circle of zombies, who proceeded to maul me to death while the chopper took off. The game credits (which show your stats for the game -- headshots, number of times healed, etc. -- in the form of scrolling film credits) were even dedicated in memoriam to me. I loved it.
And you actually earn an achievement the first time you get all four teammates safely away.
I'd love to try multiplayer co-op at least (or even versus where one team plays as the undead) but I really don't want to put up with kiddies, griefers, and microphone mouth breathers. So if some of you are Xboxing it up, lemme know.