Minecraft crashed on world open.

I had just downloaded the new snapshot (13w21a), and went to open a world. The game loaded a bit, started downloading the terrain, and then the window closed and a crash report tab came up in the launcher. I figured there was something wrong with the world, so I opened another. The same thing happened. So here I am.
Now I can't open any of my worlds without the game closing on me. At least not on this snapshot.