As a child in France: hole in the floor loo
As an adult in New York: seeing loads of adults without tattoos (where I live in Canada more people under 40 seem to have them than not); no visible homeless (wall to wall homeless addicts in some parts of my city - it's very sad but we're also sadly a bit used to it, in a visual sense at least).
Moving to the UK: we've just bought a new appliance - what do you mean there's no plug?
Washing up bowls - a bit mystifying to the non-British brain.
People just dropping by! Good god. No-one does that where I live. I got to like it, but always slightly surprised me.
When I lived in the U.K., over 20 years ago, people were more open about being racist and anti-Semitic than I was used to. Not that there was more racism, just that people did not give a shit if you thought they were nasty about Jews or didn't want their daughter marrying a dark foreigner.
Just how gorgeous much of Britain is - the architecture and the landscape. Sure there's grim bits, but overall I was in constant happy amazement at how lovely it is.
Moving back to Canada: no-one ever has a moan about work, partners, general annoyances. You must adhere to national cheeriness standards. You also must be committed to achieving and make out that your crap job is your calling. I suspect Americans might go in for that as well.
Having to relearn Canadian vocabulary. Still slip up there w some words.
I worked in an ESL college for a bit. We had to have visual signs as to how to sit on a toilet and also to put the paper down the toilet bowl, not wee and poo stained paper all over the floor (ffs, how does that even work wherever they were from?)