you really don't know what you're talking about
Well, I beg to differ.
I live in a place where more than half the people here have non-white skin, and where the guy who works in the gas station I like to go is from Senegal and is coal-black (and, for the record, the best damn gas-station guy I've ever seen- clean, efficient, polite and nice- I doubt if he'll be there long). I hear Spanish daily, and the occasional smattering of Vietnamese. And I don't live in New York or some fancy "international" city, either.
I think that the perception of the U.S. as a "tapestry" is interesting as an analogy. I agree that it's a tapestry, but there's a common thread of cultural practices and civil things that makes it into a whole piece, and it works largely because of that thread. It's probably the hardest thing for outsiders to grok, though, because it's hard to see, if you don't live here.
For example, for you foreigners out there, here are a few things about our culture that are non-apparent, and you won't see them if you're a tourist:
1. It's considered to be very rude to discuss religion with your co-workers if you don't know them very well. Why? Because we have so many religions, and even within Christian sects, it's very easy to cause serious offense. Conservative Baptists may not get along with liberal Methodists, who in turn think that middle-road Catholics are backwards, who in turn think that Jehovah's Witnesses are endangering their immortal souls. It's a very big issue here, and most people are pretty careful about it.
2. Commenting on the place of one's origin is usually seen as a source of pride, and you will see lots of people suddenly remember they're part-Irish on Saint Patrick's Day, celebrate Kwanzaa, etc., but it's even more fun if you can do both. We're all quite proud of being "mutts", for the most part. I think that's one of the reasons why so many people here liked Barack Obama- a lot of people related to his mixed ancestry and heritage. This is a relatively new thing here, mainly in my parent's generation and younger folks, but it's been a good change.
3. When people take their religion seriously enough that they wear special clothing, it's generally respected, not seen as a problem. If there's anything most Americans appreciate, it's a whole-hearted idealist. Even if we disagree about everything they feel idealistic about.
There are exceptions and caveats, of course, like in any functional society. If a Muslim refused to sell people pork, and that was a requirement of their job, few would feel a lot of sympathy if they were fired. But that's because the Muslim is trying to enforce cultural rules on others, outside our civil code and common rules, not because we hate Muslims or think they're crazy for not thinking pork is an acceptable food.
4. We're all really insincere about certain things. This usually annoys certain types of foreigner, and it's led to a lot of really stupid stereotypes about those dumb Americans with their booming voices and their fake smiles. What people don't understand is that, like the Japanese with their elaborate ways of being non-confrontational, it has a definite purpose. Every culture has these things, of course- we're just pretty deliberate about it, and train people endlessly when they're hired to meet the behavioral requirements.
Saying "thank you" and looking people in the eye when you buy a candy bar is just normal expected behavior, even though it wasn't an important transaction, and the guy behind the counter hasn't done you any favors or cut you a deal. Since for the most part we don't do a lot of haggling, the insincerity of daily commercial transactions reminds us that we're all serving each other, and that while the other person's being paid for providing that service, that doesn't mean we shouldn't be grateful. As another example, in restaurants, we leave money as a "tip", and that's an important part of giving feedback to the people who took our orders and served our food, even if we'll never see them again. Our "tapesty" has a very strong commercial thread, basically, and even if I visit some restaurant where most people aren't speaking English, tipping is still required.
So there are a billion "little things" like that that bind people together here. It's hard to explain, as an insider, because so much of it's literally invisible to me, because it's just expected, regardless of what culture or skin color I'm dealing with, but anybody who's spent a year or more here can probably say a lot more about it, both the good and the bad.
But the idea that we're all just drifting icebergs of culture that happen to be in the same place is really mistaken. We all have to work together, shop together, live in neighborhoods together, deal with traffic together, etc., and that requires a lot of interaction and tolerance and certain rules.
I'm not saying that that's perfect. Racism is still a problem, and some immigrants never feel comfortable around people who've been here a few generations. But it works most of the time.