Sixteen people list things they couldn't believe about America until they lived here. (H/t Instapundit). An interesting read. Some of the things that come up over and over are (a) how big everything is, from meal sizes, to the size of houses, to just the pure physical size of the country; (b) the importance of religion; (c) that people were polite/considerate, but not as open or quick to form friendships.
I lived in Japan for a couple of years, and I remember a few impressions both when I first arrived in Japan, and then when I first returned to the United States. And size is probably one of the biggest things that hits you. In Japan, the buildings are small, the streets are narrow and, generally, if someone had a garden (there were no yards as an American would understand the term) it was walled. But the walls were contiguous in many neighborhoods, so some streets were nothing more than cinder-block or cement walls on both sides.
For a funny story on the different concepts of distance, I remember bicycling to an appointment, and stopped to ask for directions. The person that helped me told me I should take a bus because where I wanted to go was too far to bicycle, but pointed me down the correct road. Assuming it was a great distance, I didn't even pay attention to where I was supposed to turn until after I rode a mile. Then, because I didn't see it, I road another mile ... and then another ... and then another, and started to get into an area where there were fields and open space around the buildings. Finally, I turned back and found where I was supposed to go was only a half mile or so from where I first asked for directions.
Beautiful gardens, but lousy parks.
Personal space was different. In the U.S., most people stand several feet apart when talking, whereas in Japan people stood very close to one another. It was disconcerting at first to have people invading your personal space.
The homogeneous nature of the country. There was little or no diversity of architecture or buildings from city to city. Tokyo was the exception. I only lived in Tokyo for 5 or 6 months, but I really liked Tokyo more than any other city I lived in.