You both make good points, but in fairness to rdf, I'd like to point out that while of course many countries have large immigrant populations, America's entire population -with the exception of the native peoples we tried very hard to exterminate- are immigrants or descended from immigrants, a few generations removed. And unlike, say, France - and here's where I think the crux of the matter lies - America is a relatively new country.  So most Americans' cultural identities are just as closely linked to some far away land as the country they live in.  

It's not the immigration flow that is unique, but the 1) absence, outside of museums and reservations, of a national identity preceding the immigration of vastly different groups and 2) the fact that this nation was founded - settled by immigrants who killed off the natives almost entirely - and founded only a few hundred years ago.  So for the most part, being American means being from somewhere else, by definition.  Whereas you can be 10th generation French and it means being French but you can be pro-immigration and eat ethnic food but when people ask you "What are you?" you will say "French."  An American, when asked that, will say, "Irish, German, Italian, Mexican, Chinese, etc etc."  Does that make sense?

We're not the only country of immigrants.  We're the only country with basically no native population.  Not left in tact, act any rate.

"Pretending that you already know the answer when you don't is not actually very helpful." ~Migeru.