I enjoyed that Christmas episode a lot, more than I expected from the trailers. A very nice sendoff for Capaldi.
It was interesting that it ended up involving no "bad guy" in the end. I loved the clips tying in to The Tenth Planet, and the first Doctor was funny. The characterization did't match up with the first Doctor super well, lacking the signature phrases, and he wasn't that overtly sexist, but it was still an amusing way of addressing the change in the Doctor's attitude over the course of the show to reflect the times, so I still thought it was hilarious in that way. Capaldi was awesome as ever.
I knew Jenna Coleman was going to be in it, so I was curious how they'd handle that given that he'd had his memory wiped. I liked that too.
My favourite bit was definitely how the WWI scenario tied into Christmas, and also the message of being the Doctor, as the first Doctor kind of stumbled into this stuff rather than making it his mission. Just the right amount of Christmas.
An interesting setup for the 13th Doctor, it reminded me of the start of The Eleventh Hour, except to the extreme. Hopefully the new TARDIS interior is good, because I absolutely love the current one so much. I would have liked a few more lines from Whittaker, but it was a good tease, and I have no complaints there. I'm really excited to see S11 now.
I was hoping this episode would tie up the loose end of Day of the Doctor, since you had two of the Doctors there. But that probably would have been too much to pack in.