"I really liked Gromnir's answer. Americans, perhaps more than other nationalities, do have a sense of self value. We live in a country where the "rugged individualist" is still the ideal. Dependency, on government, on family, on anything is largely disdained for good or ill. We revere and respect self made people and aspire independence in both our social and private lives.
But while that might be an American trait it really misses what Bruce was asking; what does it mean to be a citizen? I think it means an acknowledgement of community. The people who live around you share a bond with you. A shared history, culture, language, as well as geographical proximity. Being a good citizen means acknowledging that bond. Being a good American citizen means not just acknowledging that bond but also the self value (meaning individual liberty) of those people. Respecting their rights, property, privacy, etc. And this includes a willingness to defend all of this as well. If I saw someone breaking into my neighbor's home I would defend his home from that person as vigorously as I would my own and expect that they would do the same.
I've mentioned before the US Civil War is one of my favorite subjects for study. One thing that I've always found remarkable was the willingness of the Confederate Soldier to fight. Most of the officers were in the US Army before the war and were professional soldiers who left to serve their native States. But the majority of the infantry (at first) was made up of volunteers. These men were farmers and tradesmen. They were not wealthy and did not own slaves. Slavery did not benefit them in any way. So what motivated them? I read the answer in a book, a true story about a Confederate soldier called Jack Hinson's One Man War. In the book he recounts a conversation between a Union Captain and a Confederate soldier who had been captured at Knoxville. The Captain asked the soldier why he was fighting against his own countrymen. The soldier answered simply "Because you are here". That willingness to defend what is not exclusively yours, the homes, lives, and liberties of your community is what makes a "good" citizen IMO.
This shared bond is not iron. Far from it. It is the most tenuous of things and exists only so long as the self value and liberty of those who share it are respected by all who share it. The moment that is violated then there is no community. This sense of community should not be taken as an obligation to support one another. Only to defend one another. In the American example we expect you pull your own weight, support yourself. Not ask or demand we give you what we have. Yes there is some sharing to be expected but what is yours is yours and what is mine is mine. As Ayn Rand once wrote "I will die for you but I will not live for you"
Just my $.02"
this is why we can't have nice things