All 4 One is also a competent action game, from what I saw, with a series of really exciting set pieces connected by interesting platforming. But it's all about experiencing these set pieces together.
Players choose from four characters at the outset, though Ratchet and Clank seemed pretty much identical in abilities, as far as I could tell. Players have the ability to jump (obviously!), plus a melee wrench attack, multipurpose vacuum tool and a variety of Ratchet & Clank-style weapons. In the demo at South by Southwest, I had a machine-gun type item, a rocket launcher and the "Arc Lasher," an electrified whip. When two players are using the same weapon, shots fired at the same time are amplified.
In addition, players are equipped with a kind of grappling gun that can be attached to various environmental objects to create a zip line, with the second (third, and fourth) character able to grapple onto the first to create a chain of lines allowing the bottommost player to swing to great distances. Once that player has reached the other side of, say, a chasm, that player can then pull the other characters across via a leash action.
It's also possible to hold and fire another player across a chasm with the vacuum tool, and then catch up via this leash mechanic.
It's not all friendly. You're competing for item pickups and collectibles.
With methods like these, you can cooperate to travel further than you could alone (though you'll always be accompanied by at least one AI-controlled partner). Sometimes, you'll even pick up co-op weapons, like a device that connects players via an electric line that's used to clothesline enemies. You can even revive downed players by getting close to them and tapping a button. And you'll want to, since the game ends when one character dies.
But it's not all friendly. Even as you're encouraged (forced, actually) to cooperate to get through levels and strengthen player abilities, you're competing for item pickups and collectibles -- essentially, the high score. That means you work together, but you're also frantically competing to suck up as many items as possible with your vacuum. Should you feel like being a total jerk, you can pick up your partner and toss him or her out of the way.
The heavy focus from the outset on co-op adds an appeal that the Ratchet series in general has lacked. In fact, it fits suspiciously well with the series. It kind of feels like the whole franchise was designed not just to have multiplayer, but to be a co-op game, and it just took this long to make the real game.