I'd be interested in a treatment of those differences. As far as I know, a middle class is simply the class of people that earns enough money to be able to buy a house, save for retirement, and generally shield themselves from the chaos of lower class struggle.

In the US middle class means middle of the income range. In the UK it means a particular social and cultural outlook. It's almost a kind of weird extra dimension of the political spectrum. Hard to explain but you absolutely know it when you see it.