I was amazed at the primitive currency in the usa.
Paper notes and all same size and colour

When I first came to australia I thought people want to know how I am when asking: how ya doin, love?

my friend & I arrived in los angeles once, having to get around hitchhiking before we found the right car to travel up the coast. never would I have expected a cool dude in a mercedes sports car giving us a ride. but he did

I have not many assumptions as such, being more of an observer.... sorry.

Made the assumption that we would need to know the local language to get by when traveling. My first trip to Europe was France, and one of the people in our team was fluent in french. Was on a science and technology exchange. Found that I could understand science papers, especially if they had lots of graphs fairly well. On our longer trips, we have always learned some travel level language. Wife does well in german and french, and I speak some spanish.
Driving would be similar to how the people at home drive. The reality is everywhere is a little different.
Shopping in local markets might be difficult to figure out how everything works. Not so much, pointing works fairly well.

I was there twice in the 80's and got to visit 9 countries and if you see someone between 15 and 60 they spoke enough that you could get any thing or anywhere you wanted. Greece was the one exception but was told they didn't like tourists on the main land only the islands. Spent two months the first time and one the second. I've actually more relatives in France than here in the US.

I was surprised that all over Europe the food wasn't usually packaged and most showers and bathrooms were for both genders.

I was skiing with a group of Germans a number of years ago. Language was not a barrier for us as I had recently completed seven years of German language study. At the end of a great day of skiing and story swapping, one of rhe guys turns to me and asks me, "Are you Irish?" Well, my ancestry is German and Swiss, I was born and raised in America. I asked him, why he thought I might be Irish and he replied, " Well, you have an Irish accent." I thought about that for a moment or two and it dawned on me. My German Teacher. Miss Quinn, was Irish! It was the first time that I realized that other languages were subject to regional accents just like English.

From a Former Expat to a Current Expat in Thailand, I must say leaving all western cultural assumptions behind for successful living in that country is essential. You go from usually a culture of false black and whites to a culture of vast shades of grey. How I miss it.