So Downwell. Holy crap.
Ojiro Fumoto's GDC showpiece is one part roguelike, another part Metroid, and yet another part Cave Story, which is every bit as fantastic as it sounds.
The basic idea is that you control a character who can run left and right, jump, and shoot straight down while in the air. The catch is that there's a finite amount of ammo available, and it only refills when you land. The other catch is that if you die, it's back to the beginning. Pair that setup with falling down a seemingly endless (and randomly generated) vertical shaft and being able to purchase or find upgrades and you've got the makings of one great action game. And it is a great action game; the handful of minutes I got to spend playing it made that perfectly clear.
The only unfortunate thing about Downwell is that we're going to have to wait until July to get our hands on it.