[kaveri]

Language is a big factor: English is the first foreign language taught in European schools (and the mother language of most UK and Irish citizens); Americans are traditionally poor at foreign languages, so are less likely to move to a country where they need to learn one.

Another factor is culture - America is more "welcoming" to ethnic minorities, and is built more or less on top of recent immigrants. European nations have a stronger ethnic identity which is less able or willing to accept new groups.

Really though you can't compare America to Europe in a single generalization - Poland, UK and Spain, to name a few, are completely different places, more so than, say, California and West Virginia. It will be impossible (and undesirable) to achieve the same kind of unity America has, not in this century anyway.