I've been working on an Android game that I envision as a mashup of Zelda and Minecraft, so it's pretty entertaining to see Notch whip out a pretty similar idea over a single weekend.
How he maintains his focus for that long and doesn't get stumped by bugs for any great length of time is truly impressive.
And it's a real treat to see a 2D game like it being made. Not to say other participants didn't have cool looking 2d games.