In my US culture class, we had to read about US culture. The author of a very interesting article said that the US is something like a cultural melting pot. Since most of the citizens are actually not Native Americans but Irish, Spanish, French, German, Chinese people etc. it's hard to define the national character of the US. There are some features though, that fit to the majority of US citizens. However, one should keep in mind that this might not fit for all Americans and that sometimes it's a bit of a stereotypical view on US culture.
Patriotism

Competition
Competition is another very American feature. Nothing is about being good- it's all about being the best. Even their economy and politics reflect the American desire for competition. This is why especially the Republicans don't want the government to interfere in economy and want to allow free competition.
The American Dream
People who came to the US in the past, mainly came with one single goal: achieve everything you want to achieve. America...the country without limits, where you can be whoever you want to be, where a poor farmer can become president. Americans believe that if you work hard enough you can achieve anything. This is why Republicans don't want the government to help the poor. They think everyone deserves what he gets- and if someone is poor it's probably his own fault.
Prestige
Salary and profession define who you are. This is why people talk about blue and white collar workers (blue => manual labor; white => managers etc.). Women are said to be especially class-conscious. The way you dress (and especially the money you spent buying your clothes) should show others what class you are in. Usually women are in the same class as their husbands unless they earn more than their husbands. It's always about owning the newest, biggest and most expensive. Also, in contrary to other cultures, Americans love their rich and famous. They don't judge them or want them to give their money to the poor. They hold them in high esteem... the rich and famous are like role models.
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As you have probably realized, most of those American features are negative ones. I want to emphasize that a lot of them also have positive aspects. The American Dream for example made it possible for people to work their way up to a very good social position. In other countries you can never leave the class you wore born into.
Some people say that competition is the key to success. Maybe the American desire for competition is the reason why they are still the leading country in branches like technology or science.
Just like every country, America has it's positive and negative sides. But I think the cultural mix in the US makes life their sometimes very interesting, because you can meet people from all over the world.