A few observations:
The famous political thinker Alexis de Tocqueville more or less said that public opinion controls America.
http://www.gutenberg.org/files/815/815-h/815-h.htm Is this still true today? I'd say for the most part. We pull out of wars when the public gets sick of it. Gay marriage has mostly won. Marijuana is become legal in some states. Minimum wage hikes generally go through. We have a large safety net (not as good as some countries) but ours is significant. The public is becoming aware of "income inequality" and I figure something half-ass will be done. The war on drugs is losing support and I expect the laws to change. My biggest complaint is civil liberties, but most Americans seem to chose safety over freedom (at least when they are pumped full of fear).
I generally support the political theory of Pluralism.
Every once in a while we have a "critical" election.
http://en.wikipedia.org/wiki/Realigning_election
I would dispute some of your claims. Democrats won Congress on a promise to end the Iraq War and did nothing of the sort. Obama promised to pull out of Iraq in 16 months, but actually withdrew out troops when the Iraqi's said we couldn't stay. Now he's sending them back in again. He promised to get out of Afghanistan in 2014 but now he says we're going to keep 10,000 troops there. Public opinion did prevent him from bombing Syria a year ago but now he's bombing Syria.
Gay marriage has won mostly where it has been imposed by the courts. It hasn't been voted in very often.
Marijuana laws have been repealed before and then re-instated. The value of medical marijuana may help the battle this time out, but still, very few states have legalized it, and we have yet to see how long those laws will last.. Here in Florida even medical marijuana failed in the recent election. I'll grant, however, that even the failure of marijuana laws at least represents an expression of public opinion.
Minimum wage laws typically raise the minimum only to what employers are already paying. Social security and medicare are popular because people have become dependent upon them. Obamacare was not popular but it still passed.
I don't think most Americans supported the Patriot Act or the NDAA. They simply didn't know what was in them. These acts have virtually obliterated the Bill of Rights. How much of that have you heard in the mainstream media? Nada. They just don't cover that.