Re: "In an old magazine, however, I read an article by an Englishman who had traveled widely in the States. He complained that Americans spoke a common language, with few of the interesting variations found in the British Isles."

Let's not forget the distinction between language and dialect. All English speakers speak a common language. They might have to slow down a bit to understand one another, but it's still a common language.

As for the widely-travelled Englishman, unless he stopped and stayed every few hundred miles, unless he visited rich and poor, and unless he participated in many informal gatherings where people "let their hair down", then he hasn't really experienced the dialects of America.