For those of you who don't like the idea of Capaldi as the Doctor because he's appeared in the Whoniverse before, well, he's not the only one by a long shot.
Here's a great summary: Doctor Who and the Case of the Reused Actor
Also one theory floating around which I've adopted is that the doctor can sometimes pick up traits from people he's met before. So for example, when Davison regenerates into C.Baker, maybe he was thinking about Maxel from Arc of Infinity, and maybe when Smith regenerates, he's thinking about that roman guy from the Fires of Pompeii, and Bob's your uncle. And maybe when Eccleston regenerated, he picked up a Londonish accent for Tennant because he's spent a lot of time there and with Rose.