Earlier I wrote about the choice between living in a stable, oppressive state and risking violent action to fight injustice. Here we have an article stating the the United States is currently an oppressive government worthing of changing, and that people somehow seem unwilling to risk a Second American Revolution.
The article focuses more on why conditions are revolutionary (low approval rating of government, disagreement with most government policies, distrust in the direction of the nation), and offers only one reason for why revolution hasn't happened - pacifying materialism. Materialism is a fair bet for calming some revolutionary zeal, but it misses a lot of what I think is perhaps more valid.
Long-standing democracies are stable institutions. While early one, upheaval is common, and democratic governments can have a hard time establishing themselves, the longer a democracy runs, the more people are willing to trust it. We have no big revolution because some people genuinely think that electing new politicians will be all the change we need. In 2006, with a disastrous war and a general public consensus of an administration that was mismanaging the war (not to mention other aspects of the country), the change that happened was an election of opposition party politicians into the legislature. Now, with low approval ratings for the legislature (which are fairly ever-present), people wait for the next election in 2008 so that the change they seek can be made then.
Trusting that elections will fix politics is not something a large segment of the populace is willing to do, and so they tend to leave the political process and, more or less, give up their vote. This is something that happens along class lines, so that the lower ones class, the more likely they are to not vote. The lower class is generally the place successful revolutions start from, but that requires that they are politically concerned enough to resort to revolution.
There are other factors, which I'll try to run through quickly here and explain more later -
The two-party system is really good at making sure people fear the opposition party in power more than they fear inept government across the board.
The polarized electorate would rather have small gains in an inefficient system than risk what will happen if views they disagree with prevail (esp. in a revolution)
Entrenched government is really good at being entrenched, and tends to make use of fear more adeptly than 'fringe' revolutionaries make use of hope and anger.
Politics in the United States are not stated as class based, and it is much harder to make class politics meaningful in a two-party system.
Let me know if you have any more ideas.