Like any major world religion, Buddhism comes in a lot of varieties, many of which are practiced in the United States, so broad comparisons are rather difficult.
That said, from my personal experience, there is a variation of Buddhism that is largely unique to America*. It's common for well-educated, socially liberal Americans to study and explore "alternate" religious - myself included. Buddhism has struck a chord with this crowd, I believe, for several reasons:
- It provides a counter to the materialism that is prevalent in American culture
- It isn't as susceptible to the dogmatism and literalism that is common in traditional Western religions (as practiced in the US)
- It provides a spiritual path that is compatible with secular philosophies
This "American Buddism" tends to focus on self-improvement and being a responsible member of one's community. It downplays teachings that are considered too esoteric or superstitious and focuses on more tangible, everyday aspects of the practice.
Also, because it was adopted by lay people, American Buddhists tend to be very loosely organized. I have the impression that the majority of American Buddhists are practicing in isolation or as part of a small informal groups. Formal organizations still tend to be small by comparison to their Eastern counterparts and few practitioners can be considered monks or nuns. As a result, the teachings and practices have been adjusted to be more suitable for lay people.
Lastly, because of liberal bias of its adopters, American Buddhism tends to be conflated with various liberal ideals. For example, though Zen (and Buddhism as a whole) does not mandate vegetarianism, the Rochester Zen Center only serves vegetarian meals out of respect for its many vegetarian members.
* Though I'm focused on the United States, this probably holds true for Canada and many Western European countries as well.