4) References Everywhere
From outright name checks to thematic tips of the (fuzzy) hat, “Last Christmas” was a pastiche that wore its influences on a sleeve made of references. Let’s see, just off the top of my head: Alien, The Thing from Another World, Miracle on 34th Street, Ghostbusters, Inception, and, by inference, every single variation on the “it’s a dream within a dream” story we’ve seen before. Did I miss any? Almost certainly. Even better, all of them worked as plot service rather than just fan service, as it makes perfect sense that a shared dream would be littered with the pop-culture landmarks that serve as common points of reference. Plus, having The Doctor be totally ignorant of the movie Alien works as both a punchline and a variation on the notion that he can’t be bothered to remember every little detail of his centuries of life.
5) That Dance Number
It was just a nice touch, wasn’t it? After all, dancing and singing has got to be more distracting than just singing, right? (Although it really should have been set to a song by Tangerine Dream.) Alas, it doesn’t look like anybody’s uploaded a clip of the dance yet, so I’ll just leave this here instead.