If you buy into the premise that The Doctor is thousands of years old and has changed appearance and in many ways personality twelve times already why not a female Doctor? It could bring an interesting twist to the role, in any case they have been hinting at it for some time so it's hardly a surprise. As far as we know the last companion Bill, who as it happens was gay and black, presumably just to piss off certain CC punters, is gone but with Doctor Who nothing is certain.
We still haven't had a ginger Doctor yet though.