Some completely random and light-hearted personal observations:
It may be some time before people in the USA realise that Buddha was, roughly speaking, Indian.
I'm sure that most believe Jesus was a white guy from the Bronx, like in the films, or was that Spartacus, or even Tony Curtis?
Kids here in the UK speak in a patois acquired from Australian soap operas and US rap stars, and can't even recognise a potato, so not much hope for Buddhism to catch on with those who are 'down in the 'hood , blood' , unless Buddha gets a rep for bein cool wiv da bruvs, innit.
I don't think it is so much about the USA as Westerners in general, grabbing at bits of culture from elsewhere which 'float their boat' because they are exotic, like bananas used to be, or sitars in the 1960's. Once the teachings and mantras are delivered in translated form, people will either forge their own Western forms, like FPMT etc., or get turned off because it isn't mystical enough any more and head for Sufism. I mean, instead of being headed up in the pulic eye by Madonna or Tom Cruise we get really old male action movie stars in the public eye for Buddhism. Is Uma Thurman Buddhist, maybe?
I don't think American Buddhists are much different from other Western Buddhists, but I think Western Buddhists are different from those born in Buddhist countries (the ones who are not intellectuals or raised in monasteries) and who simply practice what they know, as opposed to those who love to intellectualise Buddhism or seek a saviour figure in a 'mystical' Guru with superpowers. So far I haven't seen any radicalised Buddhists, but the market is pretty saturated already and so they're mainly left with cult membership, which is simply uncool - and expensive.
California Buddhists and famous people with newly purchased tulku status are, of course, exempt from any of my generalisations - they may sue.