As a yank who lived in the UK (East Sussex) for several years, I can share the sentiments of my mates there that they believe we Americans still speak a more traditional version of the language than they do now. Specifically pronunciation of words.

For example, Americans have retained the pronunciation of the final “r” in words like “father” and “mother,” while the UK has dropped it. Americans have maintained the “flat a” sound of cat in words like “path” and “class” whereas the UK has mostly replaced that sound with the “broad a” of “father.”

It's not an exact science, but the rate of change in the language there has gone beyond the 18th century version we Americans still speak today and thus, it can be said American English, at least pronunciation, is more traditional.