I'm just dispelling the myth that the Aztecs predicted Dec 21, 2012 as the end because they didn't have any leap years, so the date they predicted has already passed.
Anyways, I personally believe we're going to drain the earth of fossil fuels, and not be quick or well prepared enough to recover and just run out. That or we melt all ice and raise the temperature. Or the sun eats us in a couple million years. Your pick.