What's the best thing about your country?

It exists.

How ya doing, buddy?

That phenomenon is called Balkanization, and this is more or less apparent in large urban areas, not quite different from elsewhere in the world, especially in Europe.

That's the price we pay for touting diversity at its most liberal sense is that we begin to lose sight of the collective being we call "American". The cultural and psychological revolutions of the past century has instilled a lot of racial and cultural identity that oftentimes trumps nationalism. The "hyphenated American" is a norm that is continually being taught, so suddenly, once again, we're more the color of our skin than our personal integrity.

Great Britain may have a better sense of assimilation because of your period as an empire, with holdings in such far-flung and diverse areas, constantly importing different aspects of their cultures into your own both in goods and in people. However, I question your assertion that Europe has a more even ethnic distribution than the United States. I read articles from the Times and other European publication lamenting on the influx of North African and Middle Eastern immigrants that have all but assimilated into the European fabric, instead segregating themselves from the rest of the Europeans in neighborhoods such as the big urban ring around Paris, or even, from another article I don't remember, of undocumented Chinese textile workers in Italy that have essentially ignored anything Italian.

I highly doubt Europe is any significantly more mixed than the United States.

Those people bringing up some laundry list of where their ancestors came from are probably doing so from a technicality, because having all those ethnicities doesn't really lend anything more special than someone whose ancestors came from the same place. This lack of cohesive identity much valued in the past was one of the primary essences of the American experience.

I can personally go off and name what regions of China my ancestors might come from, but with the exception of government paperwork, I would always say I'm an American (well actually, I would say I'm a Texan more often).

How ya doing, buddy?