There's a big difference in levels of happiness I find, as an outsider to both countries.
The last time I visited the UK i collected my suitcase at the airport and made my way to Avis rental car desk, with whom I had booked a hire car, there were 3 people sat behind the desk staring at computer screens, not one of them looked up when I stood there, I had to say 'excuse me'.........one of them managed to lift her head to speak to me, they looked like the 3 most miserable people in the world, whereas when I arrived in the States and did the same thing I was greeted with a smiley 'Good morning' and some friendly chatter about my flight and other things. Makes a big difference after a 20 hour flight.
The yanks are a positive bunch !
One big difference I dont think anyone has mentioned is the tipping culture, like the UK, we dont tip in Oz generally, in the US if you dont tip at least 15% for fucking everything, your a twat, its an annoying part of the American culture.