Taxes not included in the displayed price in the US is a big one for me.
Similarly, in New Zealand you can buy alcohol in the supermarket. Not so in Australia, although in NSW and VIC they have bottle shops in the supermarket, which is close enough.
Paying bills etc at the Post Office is apparently a very British/New Zealand/Australian thing. I’m told the Post Office in the US is purely for sending and receiving mail, not renewing your car registration, applying for a passport, doing (some of) your banking, buying stationery, or paying your phone bill.
I had a lot of trouble finding actual milk in restaurants in the US- the idea of using “Creamer” in your coffee as anything except a last resort (such as when you open the fridge for your morning caffeination and discover the milk’s off, and must resort to Coffee Whitener instead) is somewhat odd to most people in this part of the world.
A “Pie”, as any good Aussie, Kiwi, or Brit will tell you, is a pastry dish filled with meat and gravy. Not fruit. Even vegies* are a bit suss to the true Pie Connoisseur.
The legal blood alcohol limit in the US and other places seems dangerously high- .008 would basically let you knock back a six-pack of full-strength beer in about two hours before driving, by my reckoning.
Whenever you’re visiting a country that drives on the other side of the road, it takes a bit of mentally reminding yourself to look on the opposite direction first before you cross the road. 
Burger King is called “Hungry Jack’s” here in Australia. It tastes awful.
Possums are a huge pest in New Zealand. They’re protected in Australia.
The various laws pertaining to the sale of alcohol in the US seem almost Byzantine in their complexity. In most other places I’ve been, alcohol is available for sale 7 days a week during normal business hours.
*Except onion, potato, or mushy peas