"Last Christmas" was okay. I loved the overall conflict with the Dream Crabs and the multiple layers of dreams (no, I haven't seen Inception, so this was all very original to me), but the inconsistencies, such as the one above, really detracted from the suspension of disbelief. Santa and his elves were hilarious, and I'm glad that they didn't turn out to be concrete inhabitants of the DW universe. The pacing was still off. Really, the whole "slow, plodding monsters" and "long, close-up shots of people looking terrified" is not riveting and getting really old. So is the "Don't look/think/blink/breathe" mechanic of defeating monsters.
Despite my dislike of Clara and Danny, I loved the scene where they lived a happy life. And then reality started intruding and Clara rejected it, which was great. Then the Doctor appeared and she couldn't reject it, and then suddenly Danny becomes sentient, telling her it's all a dream and instructing her on how to move on with her life. Ruined again.
I think the most disappointing part was the ending. First, the sleigh ride, which made me feel like they were trying to inject the boyish wonder into the Doctor that they tried so hard to tear from him during this entire season. It felt so off to me, since this Doctor has consistently rejected the "I'm going to go explore the universe" attitude all of his previous incarnations had, in favor of "I'm going to go beg Clara to go on an adventure." Then, the old Clara scene. That was beautiful, showing that she had a good life, moving on from the Doctor and doing her own thing, a gorgeous resolution for a companion... and they threw it away in favor of a happy-happy joy-joy, everyone's-great-friends-again reunion in the TARDIS. To be completely honest, it doesn't matter to me that Clara's returning (I don't want her to, but I'm not considering that in my opinion about this). It bothers me that they ended this episode on a trite, feel-good note - so very Moffat.
Final judgment: A good enough episode. Better than much of Series 8, at least.