There wouldn't be two different English-language versions of the Harry Potter books
Actually - I think American English, with all its varying regional accents, might still have developed, but probably wouldn't have diverged quite so far from the Queen's English.
American baseball and football might still have been created, but I suspect there'd be a lot more emphasis on professional cricket, soccer, and rugby.
I'd like to think that little English pubs would be more prevalent, as opposed to sports bars.
I'd be driving an Aston Martin instead of a beat-up old Acura (a gal can dream, can't she?

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And DeltAlum - right you are, about the WMDs