I think it's fair to say that all countries and languages can be idiosyncratic, I have on occasions seen Americans poke fun out of what they deem ridiculous or unfathomable "Britishisms" that are unique to us. Even regional vernacular which would be peculiar to a particular city or area can have in another part of our small country many wondering what those expressions mean. Of course everyone is travelling about the world more these days pick up on things that seem funny or they are unfamiliar with on their travels. Sharing a common language we are exposed to a lot of American films and tv so it's inevitable that some expressions will find their way into everyday language, particularly with younger generation who adopt and adapt perhaps more than our generation have done. Who can forget when the wave of Australian tv that hit our shores probably over thirty years ago now influenced many who grew up with those programmes and it was noticeable in their adoption of the "Australian inflection" where every statement became a question.
Having read Bill Bryson's book "Made in America" quite a while ago now and as mentioned up thread, I believe there is a fair amount of old English still used in every day American English, more so than here, which struck me when I first read that as odd, because America represents part of the "New World" and therefore "modern" and yet it went forward with idiomatic English brought over by those first settlers, that became lost to us as our language evolved.
Vive la difference! as the French are prone to say, talking of whom, they are I believe very protective of their language and weren't too happy at the time when "Le weekend" and "Le sandwich" were adopted into common usage.
