First off, I'd like to mention that, contrary to popular belief, the Mayans did not predict that 2012 would be the end of the world; they simply predicted that major (global) changes would occur. Why we hyped it up to the end of the world, I don't know.
What I do know, however, is that these major changes are real, and the Sun would be the cause of them. Only in this case, there's actual scientific evidence behind it. Obviously, no one knows exactly what would happen in 2012, but what I have here is a very highly researched theory of it (supported with science!). But to fully understand what would happen then, you must first understand the sun - or the behavior of it.
The sun is made of plasma, and a property of plasma is that a portion of the particles are ionized; which is to say, they respond strongly to electromagnetic fields. Earth's electromagnetic field (magnetic north/south) is easily defined because Earth rotates in unison and at a constant rate - the sun does not. Since the sun is made of plasma (which is more like a gas than anything else), it has portions moving in one direction going a certain speed, while there will be another section moving in a different direction at a different speed. The Sun's electromagnetic fields ("fields" because there are trillions) are completely messed up and always changing; so because of the property of magnetic reconnection, bits of the plasma from the sun will release its stored energy (as a result of the flux in the magnetic field) in the form of a solar flare.
A solar flare is often accompanied by a Coronal Mass Ejection (a.k.a. "Solar Wind" or "CME" - or "Solar Storm" for the more devastating ones); those are the problem. A CME is a massive burst of plasma and electromagnetic radiation into space, traveling like a cosmic wind with speeds of up to 3,200 kilometers per second. A burst of electromagnetic radiation, mind you, is the exact same science behind an EMP. I'm not going to bother explaining what that is.
Now, out of all the CMEs that have occurred, only a couple of them have ever actually hit Earth. At least from what was recorded. September 1, 1859 was perhaps the strongest one. Because of my comparison of the solar wind to an EMP, you can guess what happened; every bit of technology on the planet short circuited (which, granted, at the time, was mostly just the telegraph) in the span of about an hour. But that was a strong one. Since then, a couple of other CMEs took their toll as well. The effects were similar: all of Quebec, Canada lost their power from one in March of 1989. In 1994, a CME wiped out a couple major communications satellites. But the effects of CMEs are greatly reduced because of Earth's electromagnetic field (the "magnetosphere") repelling the worst of them; but the larger ones - like the one recorded in 1859 - have the strength to break through that barrier and cause massive damage - even damaging Earth's magnetosphere itself; CMEs are responsible for why Magnetic North and actual North are different.
There are things in the sun called sunspots, which are huge clusters of electromagnetic fields each doing their own thing. This makes sunspots the leading cause of solar flares and (basically) the only cause of the extremely devastating ones. You can basically say sunspots are the leading cause of CMEs. Not to mention that there's one section of the sun that has a whole bunch of these grouped together.
But what does all this have to do with 2012? Well, sunspots rise and fall on the sun's surface - through convection - in 11 year cycles. The sun itself rotates (relative to the Earth) in cycles of 22 years (called the "Hale Cycle"). So every 22 years or so, that section of the sun - with all those sunspots - is pointing in the direction of Earth. Now normally, this isn't much of a problem, because sunspots aren't that active, but here's another cycle I'll throw at you: the Hallstatt Cycle. This one happens once every 2,300 years or so, and it represents when the sunspots become incredibly active. Now, 2,300 isn't divisible by 22, so the two cycles have yet to actually line up - which is to say, every time the Hallstatt Cycle occurred, the sunspots have never been facing Earth. Until December 21, 2012.
The two cycles are scheduled to be synchronous; and already, scientists observed an increase in sunspot activity over the last decade which hasn't been seen in over 8,000 years - meaning the solar storm resulting from it will be much stronger than the one recorded in 1859. So what will happen to Earth?
Well, first of all, solar winds are pure radiation from the sun - electromagnetic radiation, but still radiation - which, when administered in large doses, can be fatal to humans (or cause radiation poisoning, chromosome damage (genetic mutations), cancer, or a whole plethora of other side effects); but luckily, Earth's atmosphere shields us from the worst of it. So we're safe, right? Well, CMEs are pure electromagnetic radiation, so they'll have a tremendous impact on our atmosphere as well; possibly destroying it. They can (and already did) change Earth's magnetic properties (the whole "Magnetic North" Vs. "Actual North" thing) - some say that the one in 2012 will completely reverse the polar alignment - that, alone, will have devastating consequences. More on that in just a second - first, understand what happens to electronics in an electromagnetic storm (solar storm); as mentioned before, it's like an EMP: Satellites will be destroyed, communications will be lost, telephones will be gone, internet will be down, computer memory will be erased; all electronic equipment will be fried - unusable. But those are just the electronic effects; the same electromagnetism from the solar storm will (assuming it does reverse the polar alignment) create some extreme environmental problems as well.
Some animals have built-in compasses to help with their navigation (dolphins, whales, some pigeons, etc.) because of magnetite wrapped in bundles of nerve cells. Obviously, they have those for a reason, so if that method of navigation suddenly becomes unreliable, they might not be able to survive; they'd try to fly South for the winter, only to end up flying North (because of the reversal) and freeze to death. A change in the Earth's magnetic field would also affect any rocks with even the slightest magnetic properties (so...all of them); rock structures will slowly try realigning themselves to compensate for the magnetic change, resulting in a dramatic increase in seismic activity, causing earthquakes, landslides, tsunamis, volcanic eruptions, etc. A change in the Earth's magnetic poles would alter the movement of the Earth's core as well - it's made mostly of iron (and nickel - both are magnetic). It's the movement of the Earth's core that controls the Earth's rotation - if the poles were completely reversed, the Earth would start spinning backwards (the sun would rise in the West and set in the East), but - more likely than that - the speed of its rotation would change instead - completely messing up our system of time. Days would go by faster (or slower), distorting our internal clocks. Our perception of time would be completely changed. Years would go by at the same rate, but the amount of days in them would decrease (or increase). Maybe the Earth would stop rotating altogether - six months of darkness and six months of light - some areas getting only night or day all year around. Plants need light, and they need it in steady moderation. Nothing would survive once plants are gone.
But that's only if the Earth's poles reverse; even if they don't, there's still the very real possibility of that same solar storm overwhelming our Earth's magnetosphere, and possibly bringing about its total collapse. The magnetosphere, might I add, is Earth's primary defense against cosmic and solar radiation. Without it, the world would no longer be shielded from the natural radiation of the sun. This would make days extremely hot (a peak of about 270°), and nights extremely cold (a trough of about -240°). Pair that up with six months of night, and no electricity or communication, and the aforementioned natural disasters, then you'll be in for one hell of a time. But then again, you'll either be dead, dying from radiation poisoning, or suffering from cancer, so I guess it wouldn't be a problem for too long. Of course, many people will be able to avoid the radiation, so humans - and life in general - will still be around. But even then, they'd better be ready for Hell.