Did you catch that he considered having a long scarf and rejected it as "silly?"

I did! Alas. I think everyone could use a long scarf sometimes.

I assume that his face looked familiar because the Doctor has seen it twice - Capaldi was in the episode about Pompeii, as well as playing a recurrent character in Torchwood (John Frobisher, a definite slimeball).

Hey! He's where I started noticing Capaldi!

Maybe it was a tease, and we'll never hear it mentioned again.

I'm really hoping. Yes, we the audience know the actor already appeared on the show; it doesn't need to be a brain-teaser for the characters. The Twelfth Doctor is not John Frobisher is not Lucius Caecilius Iucundus. All separate people in their world, unless we're going to start breaking the fourth wall after fifty years. Put down the lampshade, Steven Moffat. Or at least leave it on the lamp where it belongs.

I don't think I knew before that he has a choice about his regeneration face/body. Is that new, or have I been oblivious for decades? I'd hate to think that it's the Doctor's choice to always be a white guy.

To the best of my knowledge, conscious choice of regeneration has never been an option outside of "The Night of the Doctor," and there it was only by the intervention of the Sisterhood of Karn. "The Day of the Doctor" leaned heavily on the implication that the Doctor's feelings and emotions inform the regeneration process, even if he can't see it at the time—Ten and Eleven turned out as protractedly juvenile as they did in part because they were running from the guilt and grief of the Time War, from age and responsibility and the deeply buried ways they remember it all went wrong. (See comments in link to swan_tower, above.) Which is why I think there's no mystery about the age of Capaldi's face: he's the sign that the Doctor can move forward now. He doesn't have to distract himself from his own past; he's not playing at being more innocent than he is. He looks like a cranky older man because, well, that's one of the things he is. Older, anyway. Cranky is perfectly understandable under the circumstances. I agree that there is no reason the Doctor should always be male and always white, especially in a universe that has lesbian lizard people just casually hanging out with their families in London, but since Capaldi is the Doctor we've got for the time being, I am invested in Moffat writing him well. And that includes not burying the character work under unnecessary mysteries.