Having played a bit of MGS4, I'd say the most impressive thing has got to be the game's new active camo system. The above clip does a neat job of showing that off, as Snake crawls around a plaza shifting his camo to fit his surrounds, staying suitably disguised. OH. Wait, no. That's the second-most impressive thing. The most impressive thing is how, at his age, Snake can keep his buns so tight.