1. What is the single most common business etiquette mistake that Americans make in your home country?

I believe that the most common mistake is to adopt the attitude that things like etiquette is not important, even a waste of time. Only discussions and negotiations of the technical and financial matters are of value. This view, in my opinion, reflects the assumption that every transaction stands on its own, without considering subsequent trade.
Many societies (including many Americans) do not view business this way.

2. Culturally, what have you gained both positive and negative since you have been in the U.S.?

I am an immigrant who has lived in the US for over 20 years. This question does not apply to me in the business context. Personally, I have gained tremendously.

3. If you were to conduct a seminar entitled "How to do business in your country," what cultural advise would you give to U.S. Americans?

Meet them half way culturally. Although there is truth in the saying "In Rome, do as the Roman does", you cannot give up your own culture. People would wonder why a person would do that. It may even invite contempt. You cannot insist on behaving as if you are still within your regular circle. That strike people as arrogant.

Additional Notes

Here is information on Chinese monosyllable language. There is no alphabet. Every character occupies one square on the paper and has one syllable. No exception. Thus, there are lots of homonyms in Chinese.

-- "Four" sounds very much like death; eight sounds very much like "get rich fast". Car licenses with good combinations of these characters may cost more than the car.

-- Fish is the homonym of profit. The saying is: "Every year there is profit (fish)."

-- The characters "to give a clock" are the homonyms of "see you on your final trip to the cemetary or funeral home."