The demo reportedly showed Snake infiltrating a rain-drenched compound, avoiding search lights, offing a guard, driving a jeep and then summoning a helicopter in a seamless, open-world environment. The description meshes with designer Hideo Kojima's cryptic commentary on "Project Ogre," which he described as "very free."
We can likely expect a more formal demonstration and less secretive video from Kojima Productions in the near future. For now, the Twitter consensus paints the Fox Engine, which powered the demo of Metal Gear Solid: Ground Zeroes, as something deserving of your oohs and aahs.
Update: First artwork added, with a larger version after the break.