Probably the single-most thing that distinguishes you as an American--the greeting "what's up?"
I find myself using this greeting, without thinking, to international student friends. Their response? "How do you answer that question?" Ha. To which, I have no reply. How do you answer the question "what's up?" Am I merely trying to say "hi" or really want to hear their life story? That's the funny thing about language...
My English friend's greeting is not any better-- "Are you okay?" For the first couple times, I thought there was something physically odd looking about me! I have to say, though, I have learned a lot of new "English" words from her. Every now and again, I have no idea what she is saying to me because she uses such different words than Americans! Even though we are both speaking English, there can sometimes be a language barrier...that's the funny thing about language.
I have talked to some students from Spain, and they have the same issues with Mexican Spanish (which they say isn't real Spanish!) and students from France who have the same issues with Canadian French. Language is so fascinating!