Before I get into spoiler territory, I just want to say that this is definitely the best of the four Christmas specials (five, counting the one in 1965).
Things I loved: The steampunk atmosphere. The montage of all ten doctors (New Who has taken a while to get to grips with the heritage of the series). That moment near the beginning when Tennant is actually wondering if Morrissey is a better Doctor than him. The line at the end where Ten admits that his companions break his heart. Dervla Kirwan (though I regret she played the part with an English accent, if at least with an Irish name). A Cyberman plot that comes close to making sense and doesn’t contort itself unduly for the sake of continuity.
Things I thought were a bit silly but didn’t spoil it for me: The Cyberking (reminded me too much of a Megazord, except less convincing). As eloquently puts it, the “Cyber-shades” were crap (though at least IMHO less crap than the Cybermats). The line about Rosita being Jackson/Frederic’s future nursemaid probably sounded much less offensive in RTD’s head than it came over on screen.
But all in all, really good stuff.