OMG I love this so much I don't even know where to begin! The two words fit together so beautifully! Moriarty as a Muggleborn--fantastic. And everyone so completely in character. Having it told from Watson's POV was sheer genius, allowing us see our familiar HP characters without any sort of prejudice, with the added bonus of his delightful observations of the outrageous situations he found himself in with these people. And Snape! OMG he was so incredibly impressive, and not just to Watson's uninitiated eyes.
The pacing and the plot and the mystery -- so well done and so clever! I'm not sure I'm doing this justice here, but I feel a little like you wrote this just for me, it was so perfect. And it's not even my birthday! You can sure I'll be reading this one over and over again.
Just absolutely wonderfully done! *sighs in satisfaction*