Nick Frost is, without a doubt, the best thing about this episode. He easily alternates between charming (trying to convince Clara that his presence on the roof is totally normal and has nothing at all to do with Christmas), menacing (often when talking to the Doctor, with whom he clashes from the start), and completely badass (when facing down the episode's alien menace). Doctor Who's vision of Santa is firmly grounded in modern pop culture Santa Claus tropes, but the show doesn't let itself be tied down by what the world thinks Santa should be. Frost's Santa Claus is running the show this episode and he has the breadth of knowledge, technical know-how, and simply the presence to pull this off even when he's in a room with the Doctor -- not something that just anyone, fictional or no, could pull off.
Claus and crew are a delight whenever they're on screen, bringing most of the episode's laughs and many of its best moments. Various characters try to quiz Santa on how he's manages to, well, be Santa, and he always has a quick and clever answer -- the sleigh carries presents by being bigger on the inside, the reindeer fly because he feeds the magic carrots. Even if you don't like any of the rest of it, it's a fun episode just for Nick Frost's take on Santa Claus.
Who Ya Gonna Call?
The Doctor and Clara arrive at the North Pole (at a base which looks an awful lot like the moon base from "Kill the Moon"), where we meet a crew of four on a mysterious mission... but they've been attacked by alien face-huggers, or "sleepers," who are drawn to anyone who looks at them (another familiar concept) or thinks about them. We're introduced to the premise by watching one of the crew members walking past four sleepers by dancing through the room with her eyes closed -- which not only makes for a great (and suitably bizarre) scene to kick-start the episode, but also seems to work... at least until the Doctor and Clara stumble in and draw the sleepers' attention.
To distract Clara from thinking about the monsters closing in on them, the Doctor starts talking about Danny. The Doctor intentionally pushes her buttons to provoke an emotional response, which gets her to slap him -- because, unlike what she told him at the end of season 8, Danny's dead, which means the Doctor's really hitting a sore spot with his needling. Clara's tendency to lie is apparently contagious, though, as we later find out that the Doctor also lied about having found Gallifrey -- he wanted Clara to stay with Danny and be happy instead of being tempted to run around the universe with him, while she wanted him to leave her behind without guilt... something that doesn't seem to have worked out well for either of them.