"A Christmas Carol"

Ghosts of Doctors Past, 6: "A Christmas Carol"

And so, we arrive at the most recent Doctor Who Christmas episode, and by far my favorite of all of the Who one-offs to date. What is it that makes "A Christmas Carol" quite so special? Is it the callback to Dickens, the wonderful performance by Michael Gambon, or the fact that, unlike all the earlier Christmas episodes, this one is actually about Christmas...?