I think I love Mark Gatiss' brain.
I finished watching the episode hours ago, but I've basically spent all day processing it. I think my favorite part was probably when Lestrade showed up. He gets to have a first name now! And he knows Mycroft! aqwefhoasjfgoasihfgoi;prju90jf!!!1! I genuinely had to stop the episode at that point to get up and do a little dance of joy.
Actually, I lied. I think my favorite part of the episode was the scene where Sherlock tells John that he doesn't have friends, he just has the one, and trying to apologize. omg. I've thought that these things being cannon would be lovely, but I never really thought they would be. I thought that fannish people would hope and occasionally discuss, but not that they would be an actual part of the show. Gatiss knows our fannish hearts!