I'm a children's librarian (in Boston, Massachusetts), so I have seen a lot of the UK/US variations in children's books. Mostly they are dependent on who published first, and whether or not the other publisher is interested in taking the time and effort to change the words or spelling for the "home" edition. With Harry Potter, the concern _was_ that US children wouldn't know what a philosopher's stone was, and also that they wouldn't touch the book without something more recognizable in the title. Foolish mortals.
I've been reading British mysteries and books, and watching Dr. Who, and various sitcoms for years, so very little of the vocabulary baffles me. And the children I know who have heard about the controversy think it's nonsense. As one of my young patrons said. "If I can figure out what Quidditch is, I can figure out what a lorry is too."
One of the differences I noticed is that the UK TV-movie tie-ins are better than the US versions. I still have a lovely fat book called "The Further Adventures of Oliver Twist" from one trip across the pond. And I have both the US and UK editions of a Six Million Dollar Man book. (The one with Bigfoot.) There are twice as many words in the first chapter of the UK edition. Longer ones too.