For as long as I can remember, Republicans act like dicks and Democrats act like wusses and for my more partisan friends, those qualities are endemic to their ideological opponents. Republicans are heartless: they hate poor people, gays, atheists, blacks, Jews, Muslims, and the suffering masses. It's no wonder they don't compromise and spread fear and misinformation to the electorate. And Democrats are bleeding hearts: they want subsidies for nice-sounding idea that comes across their desk and throw away the realities of the world for a fantasy worker's paradise. It's no wonder they have no backbone.
But when you add some economics, it's a strange story. Why is there no one willing to break the mold: a Republican who wishes to act even a little dovish to attract some swing support or a Democrat who's just a bit quicker to be hawkish? Such is only the stuff of fiction. The two parties must be locked in this equilibrium for some reason.
Via Matthew Yglesias, I've found strong evidence that the Democrats are a much bigger tent party than Republicans. Because Democrats have to please a larger variety of voters, they have to be more willing to compromise and just don't have that much wiggle room when it comes to policy. Knowing this, Republicans have a strong incentive to be political bullies. This isn't a value judgement; it's a law of nature. If you know your opponents don't have a lot of flexibility, you make demands (especially since there's more than a little overlap between the two parties). Thus even with majorities in both chambers of Congress and a Democratic president, Republicans can hamper or even dislodge major policy changes. It's still a bit of a miracle that health care reformed passed but it's not surprising it took as long as it did.