Everything in this thread so far is normal stuff I could have guessed. Guns, metric, tipping, etc. Most of it has large groups of people in the country that agree, or at least know.
What are some non-obvious things? Culture shock isn't about major political issues. It's about universal things that turn out to not be universal.
For example, US people have a strong culture of how standing in line works. It's basically a moral sin to butt in line unless you have someone holding your place. This is universal in the country. My understanding is that other countries differ. Is that true?
The Matrix. Blew my fucking mind the first time I saw it. It's awesome on repeat viewings, but that first watch is magical.