See, what I always liked the best about Doctor Who christmas episodes is when they manage to use the holiday traditions as a background for science fiction, which in itself is just a big idea way of talking about the human condition. When these specials really play well, they loop back and produce secular little narratives that join the speculative science and the human need for connection that can be at its most precarious during midwinter.
Spoilers for the episode ahead
Joy to the world(s?) had a lot of that between poor lonely Joy herself, the Doctor aching for companionship, the Time Hotel connecting people across time eras, and that cosmic nuclear football that seems to threaten it all.
Moffat can really pull at your heart strings when he wants, even with characters you've only known for minutes. Joy speaking forlornly to a fly in her depressing hotel room; eager to please concierge Trev, wanting so much to not let people down too much; and Anita from reception — in particular her bottle year with the Doctor was a significant parenthetical before he caught up with the main rush of narrative.
The Scooby-Doo conceit of running from door to unconnected door down a corridor holds up even under pressure of a holiday related trauma and time travel. The star seed, in all its Pulp fiction-y MacGuffin glory, isn't nearly as clever as the prefab conflict Villengard whipped up in Boom, but it's vaguely threatening enough to drive the story.
The climax is where they lost me, though. I was fully on board with Joy's loss of her mother, Trev infiltrating the star over the course of aeons, and the Doctor using the Orient Express to force a door open (a more convincing use of rope than Empire of death). And all of it coming together in Joy's ascension made perfect sense — until that reveal of the location.
I realise that this era of Who is more fantasy- and fairytale-prone than previous ones, but tying the ending into the actual f—king biblical nativity isn't cute, or topical, or even relevant. It's just pandering, and lower than I'd expect from a hardnosed atheist like Moffat. My entire family groaned.
All in all a very good special, bookending a beautiful sabbatical vignette in the midst of the convoluted action — all let down by tacked-on religious bullshit in the final minutes.