I'll try to avoid stuff you know is weird.

1. Adjectives. You can't just have a thing. It has to have an adjective. For example: Milk. I wanted to buy milk. I get to the milk section, and there's no such thing. There's x milk and y milk and about a dozen other variants. Where is the basic milk (it turns out, I wanted "4% milk") in this damned place?
2. Fresh produce. In fairness you've gotten loads better on this one after subsequent visits, but beyond some basic staples like potatoes, carrots, corn etc it was really limiting what fruit and vegetables you could get in the supermarket. Also: baby carrots are weird.
3. Your cheese is radioactive yellow. Cheese is not supposed to be that colour - but you seem expect it to be for some reason, so your producers add yellow colouring to their cheese.
4. Your eggs are weird. I'm not sure what yous guys do to to them, but it's like you blast away half the shell and are left with a porous super-white textured inner shell. They need to be refrigerated and last a fraction of the time they'd last if you just left them alone and sold them as they are laid.
5. Your bread tastes weird. Maybe it's sugar or preservatives in it, I don't know. Bread is meant to have a really short ingredients list like flour, water, salt yeast and maybe a touch of oil and sugar. Take a look at the ingredients on your bread and it's 5 lines long.
6. Portions! Your food portions are ludicrous. I'd much rather pay half the price for half as much food as they offer on the menu.
7. Money. You have this weird unconscious pecking order thing in your culture where you value people more based on their bank balance. You show a weird unconscious level of respect to someone who is rich. And similarly, unconsciously look down on someone poorer than you. Not in a mean way - just as a "I'm better than this person" way that is hard to quantify. You are aware at some level roughly how rich everyone you deal with is. I see this trait far less in people under 20. I hope there's a cultural shift on this one, because money on its own is a weird way to measure someone's worth.
8. Your police are run by the local counties. I think your schools also? I know you have state and federal police also, but most places only have police and schools at those levels.
9. I'll mostly stay clear of health, because you know your health system is weird. But I will say that it's weird that very few of your hospitals are run by government. They're mostly run for profit. Health is meant to be a government service.
10. Outside a few cities, you barely have public transport of any sort. LA is a mega metropolis, and it's train network is a joke for that level of population - something like 100 stations for 18 million people?
11. You have no idea what's going on. Most of you couldn't name the UK Prime Minister (this one has been hard to keep track of, in fairness), the German Chancellor or any of the G20 leaders aside from USA and maybe Canada/China. You don't know about geopolitics beyond whatever you guys are doing. Your world news is literally stuff USA is involved in.
12. I'll finish on a weird one: you guys are lovely. This may because I'm white and have an exotic accent to you guys, but almost everyone I've ever encountered from the USA in or out of the country has been wonderful. You don't seem to think of your fellow countrymen you meet as 'good' by default. There's a lot less connection and respect to each other than other nations I've been to.