I have only lived and visited California so baring that in mind my experience is that it is the small things...
Dreadful food in the regular supermarkets (Tesco/Sainsbury style places). You can't get what we would call normal Hovis bread. Everything is full of High Fructose Corn Syrup or regular sugar and packed with salt. Things we consider basic food items are non-existent, picked onions, baked beans and salad cream (to name a few). Bags of crisps (sorry chips) are HUGE. What we would think is a bulk purchase of individually wrapped crisps are actually just big ass bags of crisps here.
You have so much more choice of supermarkets in the US though, and far more that cater to high end food, organic, quality meat etc. You are probably shopping in the wrong supermarkets, trying going to Fresh market, world market, Market of Choice, Trader joes etc.
Standard American bread is dire, but it is hardly any worse than Hovis which i would never touch in the UK either. The god thing is even the supermarkets will sell good read form a nearby bakery. .
Pickled onions are easy to buy, I get them from cheapo Kroger, Walmart, Fresh market, market of choice.
Beans, American style beans are far superior to the tasteless UK rubbish but if that is what you want then every supermarket sells them, I picked up a tin by accident from Kroger last week.
Salad cream, yuck, why would you want it. But even that is easy to find, just go to the european or British section of your super market and fill your cart up with Salad cream, marmite and hob nobs.
Chips in the US are for social occasions. Make some Guocamole and a enjoy some chips with a fresh margarita while you watch the game.
But really, when British people complaints start off with ridiculous food items you know the person should have never left the UK. God help you if you go to Europe where your crisp selection is between salted or paprika.
Drivers are far worse than I have seen in the UK - but this could just be the famous LA traffic problems.
US drivers are terrible, but their traffic laws are also different., e.g. they have a priority form the right so when they merge to the hgihway you have to give way to them.
There are far more ghettos and down trodden areas than you realise watching US television, I know we have our own such places - Birmingham, Liverpool... for example

but here I am seeing more beggers than I ever saw in central London.
You do realize that LA is just one bug ghetto, it is like going to Liverpool and saying the UK is full of ghettos.
There have been over 700 people killed by police officers so far this year, just think about that for a second.
Yep, because the US has insane gun regulations. This is probably the #1 worst thing about the country.
Here in LA, I often wonder if that weird shaking is an earth quake, or that smoke on the horizon is a wildfire, or if those tsunami warning signs at Malibu should be taken seriously.
The UK is safe, most of the rest of the world suffers more serious natural disasters, but you still get deadly flooding and similar in the UK.
Then you get into their backwards banking and taxation system. All the utility bills that you have to pay because there is no council tax.... by cheque (sorry check

). Oh you can eventually setup online billing but they don't have "direct debits".
You have to pay utility bills in the UK as well. Almost all companies allow you to do an equivalent of direct debit, we do for TV/internet etc.
When you get your job you have to setup your own tax and do things like a W4 to best guess how much tax you should be and hope that you are right by the end of the tax year!
Which is very sensible. This way you can make sure you are deducting the right amount of tax based on your personal liabilities, e.g. how much your wife works, how many children you have, medical expenses, rental income, mortgage interest rate reduction. It is the UK that is totally backwards here leading you to either massively over pay or under pay your tax, in the US you can sort it out so you are nearly exact.
Someone mashed the rear bumper of my brand new Ford Fusion yesterday, of course no CCTV and no witnesses.
That is the great thing about the US, no CCTV everywhere.
I'm sure the US is a terrible country for not witnessing your car accident.