As smart as modern video games are, they don't know jack about the people playing them. It's a one-way street: they stimulate us, and we react though our controllers. But games can't count on players to push a button when excited, overjoyed, or scared. The new Kinect doesn't need that to read you like a book.
It can track your heartbeat, and it can tell what sort of mood you're in by the look on your face. That's a whole new playground of player input which developers are eager to exploit. Imagine a horror game that amps up the scares until your pulse peaks, then drops away before your reaction levels out. Imagine playing the other side of L.A. Noire, convincing a virtual detective that your shoddy alibi is 100 percent bulletproof. Imagine new ways to play that don't involve hopping and waggling like a ninny. We're not just blindly making stuff up, either--we've heard from developers that they wish they had access to stats on the player's mood and heartbeat, and that they'd use it to change the game dynamically.