I think the problem with this is everyone says ‘a woman. You don’t ever cast ‘a man’ as the doctor you cast a particular man. One day, I am reasonably confident, somebody in this role, whether it’s me or someone else will say, not, let’s cast ‘a woman’, they’ll say ‘that person. That’s the one that can be the Doctor.’ And that’s what you do, I think most, the vast majority of men in the world would be terrible at playing the Doctor. There are relatively few who would be great at it. So you’re not casting a man, you’re not casting a gender, you’re not casting any of that, you’re casting someone that you think will set that role on fire. For me that was Peter, before it was Matt, and someday it’s going to be Her. But it’s not about the gender, it’s about the person who can inhabit that role. The only reason you can have to cast someone in a role, full stop, the only reason, is because you think they’ll be brilliant. There can’t be any other player on the board. That’s all it can be.
Steven Moffat on casting the doctor (nerd cubed interview ~ 20 minutes)