I'm not familiar w Andrew Scott, so I don't know if he's a capable actor in other roles, but in this episode, his portrayal of Moriarty is just laughable. Not a single line was believable. Perhaps it was his attempt at conveying insanity/instability, but all he managed to do was to portray a silly, poorly conceived character with a completely amateurish execution. He by no means represented the criminal mastermind nemesis of literature.
I don't completely agree w the casting of Benedict Cumberbatch as Sherlock, but at least he's a skilled actor. It was seriously as is Andrew Scott had never acted before and was trying to be funny. It was just painful.