One of the things you learn from living abroad is just how many things you assume are natural that actually aren't.
I probably have this one a little wrong but in the states it's generally considered family comes first. Many things follow from that. For example, generally when there is a company Holiday party around the end of the year your expected to bring your significant other. If you have one and they don't come to the party you'll probably be asked about it and people will wonder if your having a fight or if maybe you aren't planning on staying at the company.
In Japan, significant others, with very few exceptions, NEVER come to anything related to the company. They don't come to any company parties. They don't ever meet you at the office or pick you up at work. To most Japanese that would be out of place and embarrassing.
When Americans generally are stumbling in their speech they often say "um". As in, "So the other day, um, I went to the store, um, and I got this new, um, green soda and, um, it was delicious." I assumed that was a natural sound, not part of English. But when I first started studying Japanese I met a girl at Berlitz. One time she was trying to talk to me and she kept using the word "eto" (pronounced ay−toe). Of course never having heard that word I asked her what it meant. Her answer was something like "you know, it's the most natural sound". It didn't seem natural at all to me. It turns out that's one of the 2 Japanese versions of "um".
Similarly, in America, if you have to lift something heavy or push something heavy you might grunt. To me, I can't really spell the sound of grunting but if I tried it might be something line "uunnah". I'd always thought that was a natural sound not something learned. But, Japanese do not grunt like that. Instead they say "yo sha" or "yo sho". Who would have thought that there were different ways to grunt?
Then again, that brings up another difference. Japanese count with their hands they start with their thumb. Americans usually start with their index finger. At 6, most Americans will hold up 2 hands. One hand with 5 fingers, one hand with 1 finger. Japanese will hold up the hand with 5 fingers and then place 1−4 fingers of the second hand in the palm of the first hand, this time starting with the index finger.
Counting Paper Money
That brings up counting paper money. I would never have even thought there were different ways if I hadn't seen it.
Many cultures remove their shoes when entering their house. It makes sense. You walk through dirt, mud, grease, oil, gum, dog poop, etc. I never really thought about that until I started learning Japanese. Why would you want to track that into your house to rub on your floor or carpet which you may want to later lay down on? It's actually more interesting to me now that most westerners don't do this.
Starch on Starch
I'm sure I'm just not thinking of a good example but mostly I feel like you match starches with proteins. For example a sandwich is bread (starch) with meat (proteins) or cheese (proteins) in it. Maybe the rule I'm looking for is at least not starch with starch so bread + fruit = okay but bread + rice = not ok. Except in Japan you can get potato salad sandwiches as well as fried potato patty sandwiches.