Some have mentioned Japanese people and face masks. Yes, they want to avoid getting tanned, but no, that's not what the masks are for. The masks are to avoid infecting others when they have a cold.
I was quite surprised that Japanese males (and generally of course Japanese manners are wonderful) would expect me to carry their luggage (as a homestay parent) & not hold doors for me. Otherwise love the Japanese (though they are not the most curious people on earth).
My son went to America last year (from Canada). He found it shockingly over-regulated, was amazed at the ads everywhere, was shocked at how little his new American friends knew of indigenous American history (down to not even knowing their local bands' names - that would never happen here as we all know whose ancestral territory we're on), general lack of knowledge about geography beyond America.
When I first went to the UK as a child (1970's), the cars seemed so small, and the coins seemed so heavy. Moving back to Canada the coins felt like fairy money.
Moving back to Canada how badly almost everyone dresses. This still surprises me - people dress like shit, baggy casual clothes with no thought to what shapes flatters or looks interesting. People at funerals in cities (eg not in logging camps) wearing track suits, jeans. People at the opera wearing jeans and tshirts (I think it's nice to have a reason to dress up sometimes).
Going to other parts of the world and seeing general size differences in people - I never used to see overweight women in England but now it's the norm. When I was a kid and visiting Germany I was a bit boggled at how large people were (as a child I'd never seen large people more than one at a time).
Personal space differences. In my part of Canada it's quite large, smaller in the south of England where I used to live. Had to tell a man off in a queue the other day for parking his lawn chair so his nose was about touching my bum. He was Canadian so no excuse for over-crowding buddy! (We were going to be queuing for a couple of hours so some people brought chairs).