While I enjoyed this Christmas Special, I second Jorpho's question.
Also, on a related note:
Also, on an unrelated note:
I missed the previous discussion here about the Doctor's change of sex. I'm looking forward to seeing a female Doctor (well, more of her than the tiny bit we got to see already). However this is an odd occasion where that's the kind of thing I'd like to see in a show, because it works in-universe: we've seen that Time Lords can change sex and even race before, so having the Doctor for once not be a white man actually makes more sense than the alternative.
But I generally don't like just sticking [insert disadvantaged demographic] into already-established characters in most cases, like black Hermoine, lady Thor, the new Ghostbusters, black Spider-Man, etc., because it just seems forced and pointy. Create new characters to increase diversity, by all means, but changing the existing ones without some good in-universe reason just feels wrong.
(I will concede that for comics characters where the name is already passed from person to person, like the Thor and Spider-Man examples, that does count as an in-universe reason, and there's no reason the next person to have the title has to be the same sex/gender/whatever as the one before. But that's already a weird trope to me. Thor is Thor and Spider-Man is Peter Parker and while it's fine if someone else wields Mjolnir or uses web-shooters, they're a different character and should have a different name).