The easiest way to describe Dead Nation is as Smash TV, set during the zombie apocalypse. Part of what makes it so easy is that it isn't actually true.
Mentioning Smash TV is going to make most people think of a top-down twitch shooter which tests your reflexes to the breaking point, and the first time you see Dead Nation, that might seem to be the case. I managed to walk out of E3 thinking that, which probably says more about me than the game.
In actuality, Dead Nation has more in common with Diablo-style dungeon crawlers than anything else, replacing treasure chests with the trunks of abandoned cars, magic swords with submachine guns, and ancient ruins with the remains of some unnamed North American city. It's like Left 4 Dead and Diablo had a freak baby.
It's a pretty standard zombie outbreak, as these things go. A mysterious disease has reduced most of the population of the United States to shambling horrors or mutated freaks, and you're one of the very few people who seem to be immune. A year after the initial outbreak, you're out of food and your shelter is under siege, so you've got no choice but to fight.
This is the kind of game that I'm surprised somebody didn't make before now, as it bolts a lot of dungeon-crawler elements onto the zombie apocalypse, and it actually works pretty well. You're vastly outnumbered, and fast thinking counts for a lot more than quick reflexes. Zombies come at you by the dozens, and you have to use any advantage that you can wring out of the environment. Every time you clear a stage in this game, you feel like you earned it.
As a single-player game, however, Dead Nation is fundamentally unfair. It's taken a great deal of influence from Left 4 Dead, and zombies can come at you from any direction at any time. If you get surrounded, you'll be dead in seconds before you have any real way to fight back, and many of the most difficult parts of the game have a seriously painful learning curve. It has a lot of choke points and obstacles that are clearly inspired by Left 4 Dead's "crescendo events," and for a few of them, the only way you'll realize the trick to it is when it kills you ten seconds after it starts.
The toughest part is the first level. Killing zombies and finding caches of "loot" gives you currency, which can be spent at intermittently-located sales kiosks for upgrades and new equipment. Exploring gives you more currency and access to better armor, which augments your strength, endurance, and agility. Once you've managed to trick out your basic rifle, acquire and upgrade the submachine gun, and stock up on grenades, Dead Nation gets about as easy as it ever will, but you should not be at all surprised if it occasionally kills you for daring to exist.
This gets pretty bad around the fourth level or so, when the giant "tank" zombies start showing up. Enemies in Dead Nation can draw a bead on you from well offscreen, and vice versa, which means the "tank" can announce its presence by coming out of nowhere without warning and punching your hairstyle in. See you at the checkpoint, lunch meat.
If that was all there was to the game, I honestly couldn't recommend Dead Nation. It's addictive in a weird sort of self-punishing way, with a heavy focus on an arcade-style high score, and once you've figured out the trick to a certain section, you're no more than a couple of tries away from beating it. Figuring out that trick involves multiple frustrating deaths, though, and a lot of swearing at the TV. It's the kind of questionably fun activity that makes you wonder about yourself, like maybe you're actually deeply self-loathing and Dead Nation is your subtle way of expressing it.
Dead Nation is seemingly balanced around the two-player mode. It makes everything about the game better. You get more money for upgrades, the hordes are easier to deal with, and every car on every street gets blown up by total accident. It'd be nice if you could independently use the upgrade station during local co-op, but that's a relatively minor complaint.
This is not a singleplayer game and it's not a twitch shooter. Dead Nation is a long, somewhat overpriced co-op dungeon crawler, and within that extremely narrow niche, it's a good time. Do not play it alone, as its solo campaign is one of the single most frustrating games I've played this year. Beg, borrow, steal, or bribe friends to play it with you, and it's a good way to kill a few hours.