I did mention that, or something very similar. But simple geography and its economic consequences do play a role in addition to the differences in mentality - they did so back in the eighteenth and nineteenth century, and they still do today. When all those Irish, Italians, Germans, whatever emigrated to the US in the nineteenth century, it was for the vast majority of them a one-way trip: at best, they could afford to take a holiday or two back to the mother country many years later, if they even wanted to, but that was quite rare. Nowadays it's a lot easier for immigrants to return to their home country on holiday or to visit family or whatever, but still it's expensive, and most Americans (immigrant or not) don't get too many days off in which they could fly back to their country of origin. So they had better make their life entirely in America and give up on the idea of maintaining close ties with the homeland.

You'll only understand the point I'm making if you then compare it to the Moroccan, Algerian, Turkish,... minorities in Europe nowadays. For them, going back to their country of origin doesn't take more than two or three days by car/ferry, at a very modest cost, and they have longer holidays, like all Europeans; so it is very common for these immigrant families to spend a month or so each year in their country of origin. And also still surprisingly common that during this time the emigrated young men and women find a suitable partner from the old home country. It doesn't take a genius to realize that this makes full integration in Europe somewhat more challenging - they are effectively citizens of both Europe and their home countries, but don't entirely belong in either.

France is a bad example actually - I've said before that France is in many ways the USA of Europe, and that includes some good sides as well as bad ones. I think they are better at ignoring skin colour or ancestry and accepting anyone who shares their republican values than many other Europeans. At least as long as said person also shares their language. I wouldn't say it's anything like the American melting pot, but, despite Charlie Hebdo and the scare stories about no-go areas where the police doesn't even dare to tread, they still do a better job at making immigrants proud of their adopted country than most other countries in Europe.

But for the most part of Europe, you're quite right. I do admire that about America and I have no problem admitting it.

You're totally right with the "large enough and diverse enough" part. In most major European cities, there are by now areas where easily 30, 40, 50% of the population is a first, second or third generation Muslim immigrant (in some entire countries, the percentages are between 5 and 10%). The scare stories are nonsense, but it's definitely true that with such numbers, this is no longer a minority comparable to American Muslims - or to European Jews. The only comparison that will make sense to Americans is with Hispanics in some areas - the kind of area where towns feel so threatened that they start passing laws or regulations enshrining English as sole official language, for instance.

I don't know if the US attracts less radicalized Muslims, necessarily. Less conservative and more open-minded ones, yes, agreed, because they are also higher educated. But that doesn't really say much about their inclination towards violent radicalism. Most radicalized Muslims in the West didn't immigrate that way - most of them are young hotheads who are actually born, or at least raised, in the West, and radicalized not under the influence of the neighbourhood mosque, but under the influence of people and writings they find online.