I just love the fact that they made the whole "Sherlock doesn't understand outer space" thing matter. That's probably one of my favorite things about Sherlock. Everything is connected.
Also I just love how Sherlock intensifies when he realizes that it's a child on the other side of the phone. That's the moment when I started crying.
Sherlock just loves children. More then that he considers himself a child. We've seen that more in series three with his relationship with Archie and his referring to John and Mary as "the best parents". And then in the last episode we see him and Mycroft both seeing him as little boy Sherlock at times. Not even knowing John was the one strapped to the bomb made him react the way that the child's voice did.
Reason number 111768 why I love Sherlock Holmes.