The only experience I have with gaming is with US and Canadian gamers, so I can't really answer that. I know that the whole D&D = Satanism former stigma was largely limited to the US, but I don't think that's much of a factor anymore, to be honest with you, except with a handful of people. The whole D&D = U R n3rd might be more universal, or it might not--I honestly don't know.
I doubt that Americans are more cliquish than people in general; in my experience human nature is fairly constant no matter where you live (not that I've lived everywhere, but I've traveled through a fair amount of Europe, and I have lived in South America for an extended period of time.) Historically, Americans may have had a more individualistic, "I don't care what you think about me" culture than some, but I doubt that's a major factor anymore, if it ever really was.