Checking Notch's twitter, it turns out he's half-assing the game's physics. Really sounds bad. Of course scripting out physics makes it run faster, but the fancy math is what makes it actually physics and not just scripted action movement. Having it all predefined and flat will stand out, especially in later stages as you build on the game. And since this is one of the base things, he'll have the hardest loving time correcting it later on. I hope he goes back to 'fancy math' before it is too late.