Call me blasphemous but I was never a huge fan of River Song. She didn't get all that much screen time so we never saw her relationship with the Doctor build. I didn't find their romance believable, so it felt more like a plot device than anything else. A cool plot device (figures, it's Doctor Who) but a plot device nonetheless. This episode changed everything. We get to see their dynamic and their romance in action, and how good of a team they make. We get to know River a bit better. Not to mention the fact that the screenwriting is excellent--this is an wonderfully funny episode.
Lately Capaldi has been growing on me, climbing the ranks and getting closer and closer to David Tennant's long held top spot of being my favorite Doctor, and, I have to say, this episode finally pushed him to the top. Capaldi's an incredibly nuanced actor who plays the role in a way that's tragic and heartbreaking, yet somehow still charming and hilarious. He makes the Doctor feel more alien and more human at the same time, if that's even possible. As other people have mentioned, this episode is really funny but really sad, with a perfect conclusion that connects back to the day the Doctor first met River. A round of applause to the writers, Alex Kingston, and Peter Capaldi. Well done.