Wow--Awesome job! I love Sherlock Holmes, and this was such a perfect crossover! I'm sure Holmes would be fascinated by magic, and the idea that Moriarty is magical is just brilliant. I loved the Holmes/Snape interactions. Also, the unstated attraction, and Harry's bumping up against the apparent wall of Severus's indifference, little realizing the true reason for the rebuffs.
I also really like Draco's tagging along--he's such a great foil, troublemaker and potential friend for Harry--really great evolution of their relationship.
And the narrative Watson voice was just perfect--many overhearings to great purpose.