They'd obviously been easing us into the idea of swapping genders with a couple other time lord characters doing so and the Doctor mentioning a few times that he'd always wanted to be a woman.
I'll give it a shot, but this is pretty dang disappointing to me. The unavoidable truth is that gender matters a great deal to a character, how we perceive them, what events and humor seem "in good taste" and so forth, and this will almost invariably have to effect the usual dynamic between the Doctor and the companions. It could certainly work, but it doesn't seem likely that it'll be the Doctor we've come to know.
Of course, the Doctor being mostly asexual when it comes to intimate matters probably makes the transition a good deal easier. I certainly hope they prove me wrong.
Overall I understand why women need more starring roles given for decades the star was always a man, but I'd much prefer that comes in the form of new characters and shows as opposed to forcing it on a character that's been a man for decades. It's not the end of the world, but it's not ideal either.
Otherwise I'm a little surprised that Capaldi isn't getting one more season, as I'd heard he really wanted one.
