Ignorance, and apathy.
There is about 30 - 35% of the country that will vote Republican no matter what. They don't care how stupid the candidate is, how much their vote will come back to hurt them personally, they're just too ignorant to understand anything except "Republicans good, Democrats bad." So the Democrats were up against that to start with, which meant an uphill battle.
But the rest of the problem was apathy. The country was fired up after the Obama victory in 2008. The country was mobilized, and it had put in place somebody who they hoped would FINALLY start doing what was right for the country. Stop the wars overseas; get the economy back on track; do something about escalating health care costs; and suspend the fascist rules the GOP had put in place about torture, abortion, and gays in the military.
Americans wanted HUGE change, and they wanted it immediately. But that never really happened the way they were expecting. Everybody had their own one or two pet issues. But despite the overwhelming majority in Congress and a Democratic President, hardly anything got done. President Obama decided to play patty-cake with the GOP, and essentially let them dictate the rules of the game. As a result, anything that got done was only done half-assed, and everyone who was so excited about the change they hoped would happen got REALLY fucking pissed off.
The man who they expected to lead this change had balked. The Congress that could have made change happen was not willing to play hardball with the Republicans. So nothing happened. And when the Democrats went looking for support in 2010, the base stayed home. Why, after all, should they support a party that was only giving them lip service and instead just spanking their puds in Washington D.C.?
This election wasn't about some huge wave overtaking the country, the Tea Party got its ass royally kicked, and turnout was staggeringly low compared to 2008. This was President Obama's base sending him a wake-up call --- NOT the population at large.
The message to President Obama is: end don't-ask-don't tell immediately; get unemployment under control; re-regulate the banks and investment houses so they can't fuck us again; take corporate influence out of American elections permanently; make corporations pay their fair share of the tax burden; and make universal health care the law of the land. Do this, and we'll be back in 2012. Do not, and you'll suffer the consequences. Not that you'll lose in the general election: you'll never make it past the primary.
Americans spoke loud and clear on November 2, all right. Change is what they asked for in 2008, and President Obama has less than two years to deliver. Or else.
