Brit in America.
Toilet seat covers in US public toilets (especially the clingfilm type ones which slide from one end of the U-shaped toilet seat to the other when you flush, only usually the mechanism is broken so the toilet seat is covered in bunched-up clingfilm)
Ice taking up most of a drink. Incomprehension at a drink without ice. I was hugely jetlagged at an airport once and had a rant about my smoothie not being a smoothie because it had sharp bits of ice in it.
Needing ID to pay for anything with plastic. Or to order alcoholic drinks even if you're 75. And a US passport doesn't count as ID... but passports are OK for foreigners even though staff will have even less clue whether they are fake!
'Drink' only means alcoholic drink, except when it doesn't. So when it was 35 degrees DH and I agreed we were desperate for a drink and cousins looked disapproving and said 'bit early for alcohol?' We only wanted water!
Rural America:
Effects of being so spread out - photocopier engineer arriving by helicopter, milkman comes once a week and would leave 144 pints (cardboard cartons, a gallon each) in the garage. Learning to drive and the instructor not understanding why I slowed for a level crossing. ('It's a railway!'). Turned out there's two trains a year each way, for the harvest. Giving 14yos driving licences with any excuse. Thinking a 15yo is much safer driving anywhere than walking to school.
Paranoia about alcohol, local paper headlines about students and their parents getting jailed when some 20 year olds had a party that got a bit wild, not because of the noise disturbing the neighbours, but because alcohol was being drunk in the kids' own home!
But the main culture shock is just how the Government is seen as 'other' and imposed, and to be resisted at all costs, rather than hoping 'our' lot get in. Which leads to acceptance that you have to choose between which prescribed meds to take, have to apply to charities to get hearing aids, and ongoing treatment like physio just isn't a consideration (hence the opioid epidemic) even with fairly good insurance. And the Flint water scandal again just being accepted as one of those things.
Church being the default place to meet a long-term partner - I find it hilarious when my cousins hit their late 20s and all start going to church regularly for the first time since leaving home. Once they find someone it drops off and then they mostly stop completely after marriage.
All very different to New York and other US cities.