Dual citizen here. Absolutely impossible to generalise about the US, it's too big and there are too many people.
Healthcare: If you have the money, or a job which provides good coverage and access, fine. Otherwise it's a mess. It's also very profit-driven, with providers tending to push you into treatment options which benefit them. The constant commercials for drugs are one of the first things I notice when I visit. My main issue with Obamacare is that it embedded the parasite in the system, and will, I think, prove very expensive for providing nothing like universal care. The "cheap" policies have such high deductibles they are practically worthless.
Crime: I guess there are lots of guns around, but I rarely saw one in my 30 years there. Was mugged once when I lived in New Jersey, but that's a rite of passage. Never met any of the Sopranos.
Culture: You can find pretty much whatever you want, though thousand year old castles and cathedrals are thin on the ground. As far as TV goes, there has been some very, very good stuff produced in the States along with the drivel; just like the UK.
The car culture: Given the size of the place this was probably only natural, and the UK is equally car-mad (but thankfully mostly open to ramblers). When I go back to visit my hometown in the midwest, however, what I most love is riding a bicycle on those big wide quiet streets.
Politics: As maddening as the UK, just in its own way. There's some beautiful writing in the founding documents though.
Housing: My sister lives in a house which cost the equivalent of about £80k. I would classify it as a mansion. Set it down in East Sussex where I live and it would easily be 10x that. I know it's largely about space, but still, it highlights just how insane prices are here.
Other stuff:
The IRS. Ugh. Taxes are far more complicated over there, and wherever you go in the world, as a US citizen you cannot escape the Internal Revenue Service. I won't go into the details, but dual citizens are now facing such onerous filing requirements and difficulty opening foreign bank accounts that more of us are contemplating renouncing our citizenship (and renunciation itself isn't free; incredibly, this year the price went up from $450 to $2350). Just something to consider if you're ever offered citizenship but don't wish to spent the rest of your life there.
Patriotism. The UK has its monarchy; the US its flag. The former is, in my opinion, fundamentally wrong; the latter, the banner of the most violent country in the world (it's their chief export). I don't want my taxes going for endless undeclared wars.
@jollygoodthen, not feeling very jolly at the moment...