I just blundered across this on a Guardian blog:
But the thing about Doctor Who companions, whatever their age, background and ethnicity, is that they are all, essentially, the same character – the big-hearted Earth girl who finds the hero within herself. That's the story of Doctor Who, and what did for the original series was when they strayed too far. After all-time-favourite Sarah Jane, the Doctor travelled with a tribeswoman, a Time Lady, a genteel alien lady, an Australian air hostess, an American, and Bonnie Langford. It took until the rigorously badass Ace for them to get it right again, and by then it was too late.