How fat everyone is. You get used to it in the States, but when you first return it's the most immediate thing to notice. Second thing is the amount of billboards.
I’ve never eaten meat, so I never knew, but my husband checked every packaged sausage in four grocery stores (I stg that’s what he wanted to see) when we visited recently and they all had high fructose corn syrup. I’m relatively defensive of the use of hfcs over sugar, but I don’t know how people justify it in sausage (I would even forgive one with barbecue sauce in it or something, but an unflavored sausage shouldn’t be sweetened!)
It's culture shock leaving the coast and travelling 2-3h inland in the US even. People in the coastal cities tend to be normal human sized, but you go inland and they're shaped like bowling balls.
I've spent 3 years in Japan, 4 years in the UK, and a year in Korea. For me it was how incredibly rude everyone is. Sure I encountered rude people in other countries, but it's on another level here. Also, there are so many bad drivers that drive so confidently here.