Spoiler Level: High
First off, major major THANK YOUs to BBC America for running this on Christmas day. That was truly awesome of them. I hope it got good ratings and made it worth it for them.
When I first heard the concept for this episode I wasn't too impressed. I kind of felt we had already done a story with Dickens and Christmas and ghosts with "The Unquiet Dead." But really, now... I should know to trust the Moff.
Best. Christmas. Special. Ever!
Surprise hit me right out of the gate with the opening shots, when I realized that this wasn't taking place in the past but in the future on a different planet. Yay different planets!! And Amy and Rory are on their honeymoon on a spaceship! Yay spaceships!! Of course, it's a crashing spaceship, and from there comes our plot-- the only one who can save the spaceship is the Scrooge of the story, Kazran Sardick. And of course he can't be bothered, writing off the 4003 people on the ship as "surplus population." So it's up to the Doctor to change his mind.
I've said it before and I'll say it again: No one, but no one has used time travel as a central story device in Doctor Who like Steven Moffat. He always makes it a critical part of a story, and wow does he do it with style here. I expected the Doctor to take on the role of the Ghost of Christmas Past, but I figured it would be a more conventional method of him taking Sardick in the TARDIS to view his own past; but no no no, that would be boring. Instead the Doctor shows Sardick home movies of his past, then travels into Sardick's past and changes it, so Sardick gets to watch it unfold both onscreen and in his new memories at the same time! Now that's using time travel to its full potential! And that's what I love about stories written by Steven Moffat. A hologram of Amy becomes the Ghost of Christmas Present, and I won't even dare say here what he comes up with for the Ghost of Christmas Future. It's clever, it's moving, it's emotional, it's time travel, it's Christmas, it's just perfect.
For 31 seasons, Doctor Who was a show that used time travel as a plot device to set stories anywhere in time and space. When Steven Moffatt took over, he made it a show about time travel, and as such has done what Doctor Who does best-- reinvent itself every so often to keep it fresh. "A Christmas Carol" is the perfect example of the new, current Who all in one episode.