Social categorization

In the UK, arguably the most salient social categories are accents. There are broadly three: no particular accent suggests middle class. Strong regional accent suggests working class. And "posh" public school accent says upper class.  

The US doesn't rely on accents as much as color and language. The three broad categories in the US seem to be white, black and immigrant. Black and immigrant are easily identified by their color and the two are separated by language. 

Language, while not immutable, sometimes leaves speakers with a national accent that suggests the speaker is a "foreigner". But distinguishing accents, which can be eradicated or acquired, color cannot.  Social categorizations in the US, based as they are to such a degree on race, are thus more immutable than in the UK.