Mendo,
You stated that "nothing is worse than an tourist trying to dress like a local." If we were talking about djellabas or berkhas, turbans or kimonos I would agree. However, I don't think it's unreasonable to dress in long pants versus shorts, wear shoes rather than flip-flops, or wear collared shirts rather than T-shirts offering free moustache rides.
I had my first and worst run-in with Japanese tourists at the Saint Chapelle in Paris. Signs prohibiting flash photography are prominently displayed all over the entrances to the church. These signs are in numerous languages, including Japanese. A large group of Japanese tourists came off a tour bus, swarmed the chapel, pushed everyone else out of their way. Then, a veritable barrage of electronic flashes fired off from forty cameras in rapid succession. The tourists then climbed back on their bus and departed before monument security guards could do anything. Keep in mind that the prohibition against flash photography is for the protection of ancient works of art; camera flash will fade old dyes and pigments. These Japanese tourists took pains to violate a very common sense rule of behavior. The American and Canadian tourists we encountered were friendly and respectful, even stepping aside to let the swarming Japanese pass.
Also while in Paris we encountered a group of rowdy German men who were literally molesting a young French girl. I don't speak much German, but I do know the word for "c*nt," "futze," which was being loudly called at the victim. Fortunately, a large group of French men quickly surrounded the German louts and beat the snot out of them.
Most Americans I have encountered while traveling have been polite when dealing with locals, but have been overly loud and obnoxious when "partying." The rare occasions where Americans havde been publically rude were inevitably fueled by alcohol. Lots of alcohol.

OTOH, Americans seemed more patient while waiting in lines, tried to speak the local language more often, dressed and behaved better than their tourist counterparts from other countries. I agree with Mendo that much of this hype spins directly from U.S. foreign policy and that we have very little to do with changing attitudes toward American tourists.
By the way, if comparing male to female American tourists, I can categorically state that the women are usually much more rude, impatient and irritating than the males. Men tend to roll with the punches a bit; we handle waiting better, will be more diplomatic in restaurants and bars, and generally are better behaved than our gringa traveling companions. Wonder why?
I think the secret is to simply dress comfortably but clean, speak a bit more softly than usual, and continue to stay relaxed, even if you're impatient or irritated. I do believe that each of us is an ambassador for the U.S. and/or Canada, and that we do have a responsibility to put out our best appearance and behavior.