First of all, we have to dispel the myth that the Mayans predicted apocalypse for December 2012.  There is absolutely nothing correct about this statement.  The Mayans had a cyclical concept of time, and at the end of their Long Count calendar, the count of days would be reset - just like an odometer rolling over to all zeroes.  An interesting event, and presumably something of a paperwork hassle, but not the END OF DAYS.

In fact, the ancient Mayans themselves were not in agreement about what the end of their Long Count calendar meant.  The general consensus among archaeologists was that it would be a big party.  The concept of "apocalypse" was not spoken or alluded to.  And in fact the Long Count is only one of several calendar systems that the Mayans used.  

Contemporary Mayans (there are some Mayan tribes surviving in Guatemala) have no interest in or fear of 2012.  It's not a "thing" for them.  Because the whole thing was cooked up by some nutty Westerner forty years ago, and it has nothing to do with the actual Mayans.

A lot of this Mayan nonsense can be placed squarely at the feet of John Major Jenkins, who Wikipedia describes as an "independent researcher."  Truer words have never been spoken.  Jenkins has long championed the connection between the end of the Mayan calendar and the solstice, as well as a random kettle of other coincidences.  (A similar "pattern" could be found for any given date at any point in human history, of course.)

To say that the 2012 prophecies are "unscientific" is to be entirely accurate.  Jenkins is quoted as saying "Modern profane science is the degenerate descendant of an ancient sacred science that long ago perceived and embraced many dimensions of reality."  In the case of the Mayans, I guess that means that Jenkins believes that sickness is caused by "the captivity of one's soul by supernatural beings."  The Mayans also correlated the color of herbal cures with the color of the illness, therefore prescribing a yellow plant for jaundice, and burning "feathers of red birds in curing yellow fever."

It's difficult to address any of the specific modern 2012 prophecies with a straight face.  Each is more absurd than the last.  For example, the idea of a "Planet X" or "Nibiru collision" is patently ridiculous, but that doesn't stop people from believing in it.  It's worth noting that this idea originates from a woman who claims that she received the information from the aliens of Zeta Reticuli (Note: not an actual star system) via a telepathic transmitter in her head (note: insane).

The fundamental problem with the Nibiru theory is that Nibiru cannot possibly exist.  It cannot follow the trajectory "predicted" for it, it would be visible to the naked eye, and everything Nibiru is said to cause (i.e. stopping the rotation of Earth) violate basic physics.  Not even the fancy physics - just the regular stuff, like what you'd see on any given pool table.

As for the various "galactic alignment" theories, there are two problems: 1) none of them are true alignments, and 2) even if there were going to be alignments, nothing would happen.  Surely we have seen enough of these supposed cosmic alignments to have learned that when things happen to line up (as, in a complex system like the universe, surely they sometimes must) that doesn't mean that God (or worse, neutrinos) descends from Heaven and smites us all.