Activision thrust the respective controllers in our hands and let us go nuts. Each demo began with your foursome (Cap'n A, Thor, Spidey, and Wolverine are the defaults) storming into a heliport to help Nick Fury. Like usual, most helping comes in the form of breaking stuff, though you will push and pull crates and activate switches.
The environments are extremely destructible, to the point where we could easily jump through busted walls for shortcuts. Various mid-bosses pop up, including Scorpion and Bullseye. Talk-trash though they did, the odds are severely against them, and they end up beat down at the hands of our heroes. We earned some gear for our trouble. It wasn't a total cakewalk, but there was nothing we legendary heroes couldn't handle.
After Nick Fury and Black Widow were safe, the demo shifted to a major boss battle: taking on the massive Ymir in his ice domain. We were the ones getting spanked here, as it's apparently quite far in the game - when our characters would be 20-30 levels higher than they were here. One secret debug code later, and we were invincible.
This boss fight is very reminiscent of something out of God of War. The huge Ymir unleashes massive attacks, while you battle other minions. Fallen foes drop huge spears, which then must be thrust into Ymir's back. This is not easy, and requires either rapid-fire button presses on 360, or controller sweeps with PS3 and Wii.