What should I absolutely not do when visiting your country?
(Originally posted on Quora.com)
Being inconsiderate One thing Japanese hates the most is someone who is inconsiderate of others. Japanese are (sort of) educated to be respectful and always behave putting them into other people's shoes. So seeing a person who is inconsiderate to them, regardless of where he/she is from, would touch their nerves.
Because Japan is a society where over 90% of the people are the same race (Japanese), everyone is expected to know the standard customs and behave in the same manner. In other words, the majority of Japanese, "unconsciously" expect non-Japanese (both living and traveling) to know their customs and behave as much as they can.
That being said, knowing all their customs is rather challenging (even I don't know all of them). So, what I suggest is to do this: if and when you encounter a situation where you feel is awkward (judging by the face of the Japanese person in front of you or the atmosphere), then, simply say "Sui•ma•sen" or "Gomen•nasai" (I'm sorry). (tip: the more you say it, the faster the issue will resolve.) This will show you are trying to be considerate.
Engage in loud conversation when riding public transportation (exception: while in Osaka, people are more acceptable to talk loud on trains and buses.)
Enter someone's home without taking shoes off Japanese culture separates the home from the "outer world." As such, taking the shoes off when entering someone's home physically signifies this notion. While most Japanese understand that many westerners have no such custom, everyone is expected to take their shoes off when entering a home. Also, there are some public places such as temples, shrines, and restaurants where one must take the shoes off and some of those places have no signs to tell people to take their shoes off, so non-Japanese visitors need to pay careful attention.
Engage and show deep affectionate behavior in public places Traditionally, kissing in public has been considered taboo in Japanese society. In recent years, however, especially among younger generations, this tradition is changing. If a Japanese sees a non-Japanese couple engaging in kissing, hugging in public places, they would understand and accept that the couple may come from a different culture. The problem may arise if and when the couple continues kissing and hugging for say, five minutes in the public eyes. There is a level of tolerance, and when a non-Japanese person crosses the line, some Japanese may get uncomfortable, or even get offended by such behavior, though in reality, no one would say or tell you to stop.
In case you decide to go to Mixed Sex/Coed Onsen as a couple, while you're bathing, it's best to avoid kissing and hugging at all, simply because, well, let's just say Onsen is different from Jacuzzi.
Show anger, engage in an argument in public places They are something most Japanese would not do in public places. The main reason is that Japan's culture is shame-based. Japanese despise being embarrassed and publicly humiliated and avoid both at all costs. As such, showing one's anger can be considered as a sign of low intelligence, as an embarrassment.
Ask for substitutions on a menu at a restaurant While this custom is pretty common in the US and some other countries, to do so in Japan can cause a commotion and many Japanese would consider it as quite "inconsiderate" or even "offensive."
Making a mess or not cleaning up your own mess Though there are some exceptions such as summer beaches, concert event venues, and sports arenas, you will find out that Japan is a very very clean country, especially inside of public buildings as train and bus stations, city streets, restaurants and most notably, public bathrooms. Not only everyone is expected to keep them clean, but also, to clean up if you happen to make a mess. So, it is advisable not to throw away trash on the street even if that's a tiny piece of paper, especially when you are a non-Japanese person (i.e. Gaijin).