Well, the Cybermmen are back as primary vilians for the first time since The Next Doctor (only the fourth story of the revived series). Viewers also see Cybermats for the first time in 36 years. It works very well for the storyline, especially since Cybermen have a history with the Doctor that has not been overworked.
The return of Craig (but not Sophie) also worked well, especially since Craig and Sophie have a baby boy named Alfie, who prefers to be called "Stormageddon, Dark Lord of All." Craig is genre savvy enough by this point to realize that a visit from the Doctor means trouble, and decides the safest thing to do is to follow him.
My favorite part of this episode was all the scenes between the Doctor and Alfie. This Doctor, the baby-face Doctor, is the one who can't stand to watch children cry, and his conversation in Alfie's nursery is absolutely heartbreaking.
But I am old, Stormy. I am so old. So near the end. But you, Alfie Owens. You are so young, aren’t you? And you know, right now, everything’s ahead of you. You could be anything. Yes, I know. You could walk among the stars. They don’t actually look like that, you know — they are rather more impressive. [uses his sonic to make a starry sky appear on the ceiling] Yeah! You know, when I was little like you, I dreamt of the stars. I think it’s fair to say, in the language of your age, that I lived my dream. I owned the stage. Gave it a hundred and ten percent. I hope you have as much fun as I did, Alfie.
But the most surprising bit was the last five minutes. I was not expecting to see River, and the Silents...It was bad enough when I thought young River shot the Doctor, but for it to be the River we know and love, forced into the suit and unable to stop herself(as it heavily suggests)---oh man!