Comments behind the cut. Somewhat spoilerific.
It was mostly just meh. Lots of talking, a hell of a lot of trying to be deep and meaningful. More appropriation of canon history - the First Doctor not wanting to regenerate, rather than his canon dignified acceptance of his death; the identity of the captain to wow the audience (couldn't they have referred to some other important character, rather than bringing him back yet again?); more explanations of why the Doctor left Gallifrey. Scenes of discrimination against Bill and/or mentions of her orientation, since that was a requirement of every episode this season.
"Ponderous". That's the word. The show has been ponderous for at least the past season, if not all the way through Twelve's run, and that's what this episode was.
All right, enough whining.
Mostly, the plot was pretty weak. It basically served the purpose of putting obstacles in the Doctor's way so that there was time for the characters to make long monologues about purpose, fairy tales, and memory. Its main goal was to provide both Doctors a reason to regenerate, and, well, I'm hard pressed to actually state what did it: something about the Christmas armistice showing that good things do happen sometimes, and that's worth living for?
The Testimony was interesting and cool, but waaaay OP. (That's "over-powered" in gamer lingo.) Moffat just gave the show the ability to bring back any character ever, for no reason and with no consequences. But again, that fits his storytelling patterns.
It wasn't a bad episode. It just wasn't good. Oh, except the Doctor's final speech was just horrid. For me, it encapsulated everything about the past three seasons: Peter Capaldi given the worst material and just running with it. He's really a fantastic actor and his Doctor was great; he deserved better.
My husband couldn't help but joke that "Oh, typical: new girl Doctor is a bad driver." :) I wish we'd gotten to see more of Thirteen here - all of the previous Doctors got a few lines to say, even Ten -> Eleven, who also regenerated alone. We got a nice glimpse of who Eleven was in his first appearance. But I will say, I was very pleased that Thirteen saw herself and smiled. I've been apprehensive that the first female Doctor would be plagued with adjustment problems and that doesn't look likely anymore.
One last weird thing... During the very short scene between One and Twelve where they're talking about why they don't want to regenerate, I realized that the music in the background was familiar. I thought about it for a moment and identified it as a new, soft and sweet arrangement of the music from "Doomsday", the iconic music from the scene where Rose dreams that the Doctor is calling her to Darlig ulv Stranden, and then their meeting there. Such an odd choice! I still haven't figured out the significance of that.