The first thing that springs to mind is that American accents, to an English person, generally make the speaker sound more outgoing, confident and a little bit stupider (there, I said it!).
P.S. Not nearly as stupid as the prevailing estuary English accent you hear in home counties/London schools these days.
Edit: On reflection, when I think of an American accent I usually think of high-school girls who punctuate their sentences with "like" and raise their tones as if they're always asking a question. So I think that's where my prejudice comes from!
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