Moriarty in mortal form is just that, a mortal man. It was the mystique of a mastermind pulling the strings behind the puppet that made him bigger than life, and in turn a comprable opponent for Mr. Holms.
Afterall, The Great and Powerful Oz was just a simple man once the curtain was pulled back as well.
In actuallity, and in part the reason I think Sir Arther killed off Sherlock, Mr. Holmes was reaching a point of superhuman powers and was becoming harder to relate to. The entire principle was built on Sherlock as a man that just saw things differntly than most and his "simple" explanations made you feel like anyone could deduct like he does. (even me
).
Moriarty helped bring Sherlock down to earth and ground him as nothing but a man. A man who anyone could become if they just took the time to look at the world a little smarter.
Of course this is just MHO, and I could be wrong.
