I have traveled alot and I do notice that it is a small number of Americans who catch the attention of the locals in a negative way. Some of them are considered rude when they are abroad, and I'm sure those are the same people who are considered just as rude at home. Yet, those are the few that contribute to the bad rep Americans get abroad.
As far as observed mannerisms go, I notice that American women (depending on where they travel to, like Southern Europe, or Asia) tend to be considered less graceful than the local women, simply because American women may not adhere to what the culture finds "feminine". It could be the gait, it could be sitting posture, volume of speech, presentation, clothing, amount of skin showing, hair length, whatever.
I think wherever we happen to go, we have to remember a few things:
__________________________
-Know what not to do in the place you're in. Learn an acceptable way to apologize if their has been a faux pas committed.
-Try to learn at least basic conversational bits in the local language(s). People can get miffed with a quickness if you come at them in the street expecting to exchange information in English. I understand that when travelers are lost or stressed, it slips out and it happens. However, I don't know anywhere in the world where that is considered proper.
-"When in Rome, do as the Romans do."
-The place you visit is arranged to make the locals feel comfortable, not to make 'you' feel comfortable.