My answer is along the lines of lostdogrwd's. The world AS WE KNOW IT will end...but not on a specific day of a specific year. In fact, it's been "ending" for years, we just been ignoring the changes until they became too obvious to ignore.
Global warming IS a reality and has been for decades, but that doesn't mean the planet will explode or we'll all die of heat stroke. What it DOES mean is glaciers and polar ice caps are melting, same as they've done many times in the past but we weren't around to notice or be alarmed.
At one time, the polar ice caps were where the equator is now. At some point, climate conditions caused Earth to shift on its axis. "Flipped". Axis shifting WILL happen again, but not in one day. It's not like you'll go to bed after the sun has set in the west and wake up the next morning to it rising in the north (or what used to be "north"). It doesn't happen like that.
But we ARE accelerating the process with our addiction to bio-fuels and uncontrolled harvesting of old-growth forests both for the manufacture of wood products or simply to get to the oil under those forests.
Politically and socially, the world **as we know it** will end because it's becoming apparent the systems currently in use don't work anymore.
As for the Mayan calendar, I think it only goes as far as whatever method of calculation they were using would allow. Perhaps the poor souls counting "how many days are left?" simply got tired of counting (or died of old age)! ;D