I have to say this has to be my favorite Christmas special and my favorite episode including Capaldi. I adore River Song and that ending was perfect! I really enjoyed the dynamic between capaldi and river, much more than capaldi and Clara; and think for the first time the episode balanced out humor and drama well. Yes it wasn't a villain centric episode, but I still enjoyed learning more about the relationship between the doctor and river. Her speech about how she loves the doctor and how she felt he didn't love her back was beautiful, and I seriously would love to see her back. I'm still hoping that the doctor does find a way to save her and bring her back as a companion every now and again...but I don't see that happening. I think if this was river's final episode I would be satisfied with her end because there was closure and we all know that she is heading to the library next so it does complete the cycle....
Overall I am really pleased with this episode and am finally enjoying watching capaldi as the doctor!!!