I do not believe the election proves much other than that corruption scandals and the Iraq war hurt Republicans. Posner gives a very good discussion of some of the criticisms made about the American system. I will comment on a couple of the issues.
I have seen no convincing evidence that past limits on campaign contributions have improved the political process, or weakened the significance of interest groups. A major problem is that campaign limits have been difficult to enforce. Also, as Posner indicates, the Internet has opened up the possibilities of raising large sums in small individual contributions. I have doubts too about whether it is better or fairer when some groups use lots of time of young people and others to help in campaigning rather than money.
In addition, some of the most powerful interest groups have not mainly had rich members, such as farmers, the teachers union, craft unions (at the local level), and groups in favor of sharp restrictions on immigration. Very rich Americans and large American corporations have political power to be sure. Yet the tax on corporate profits in the United States is much higher than in most Western European nations, and the American tax on inheritances is far from the lowest among rich nations.
The United States does have greater earnings inequality and proportionately more rich businessmen than European countries or Japan. This is partly because the United States has higher before-tax returns to education and other skills than these other economies, even though a larger fraction of Americans get a college education than in most European nations. The main explanation for the difference is that the United States has a much more flexible economy than most other nations. In a knowledge economy, this produces bigger benefits to greater education and other skills. It is also easier to become an entrepreneur in this country than in most other countries.
Political scientists have long wondered why anyone votes in a democracy since any individual's vote is very likely to have a minuscule effect on the outcome of a political race. The same logic implies that voters have little incentive to be informed about the issues, even aside from the fact mentioned by Posner that many of these issues are highly complex and difficult to understand--such as the effects of a federal budget deficit on the economy. This means that many votes, particular those least committed to voting, are likely to be swayed by political advertising and emotional appeals. Under these circumstances, it is not obviously advantageous to have large turnout rates.
International comparisons of political outcomes in Europe vs. American suggest that Europeans put more emphasis on equality than Americans do, and less emphasis on efficiency. These and other differences might be attributed, although causation as usual is tricky, to the fact that European nations have much sharper restrictions on campaign contributions, less opportunity for gerrymandering, generally greater voting participation rates, and apparently better "informed" voters than in the United States.
Although the Europeans have less inequality typically, they tolerate much higher rates of long-term unemployment than America has. All studies show that long-term unemployment is the most destructive of self-confidence and measures of "happiness". European countries protect agriculture against imports from poor nations more strongly than America does, has a poorer environment to start businesses by individuals with limited resources, and generally have policies that are less tolerant of immigrants from the third world. The European social security system that provides retirement income and unemployment benefits is much more generous than the American one. However, Europe spends a lot less on health, including the health of the elderly and the poor, than America does.
Which system is better: the American political system, or the European model with lower campaign contributions, few opportunities for gerrymandering, larger voter turnouts, and apparently a politically better informed population? One can differ on the answer, but it is far from obvious to me that the European approach works out better in practice.