Aside from his lightning-quick deductive technique, his vast knowledge of ephemera and his astounding ability for disguise, Holmes is also compelling because of his ‘dark side’.
He is an opium addict, has traits of the manic depressive and is cruelly frank. He is arrogant, efficient and a genius. He is, overall, a fascinating, complex character with as many vices as virtues.
From his powers of observation, to his expert-like knowledge of a diverse range of subjects, to his masterful art of disguise, to his somewhat surprising physical strength for a man of his build and stature, Sherlock Holmes possesses many of the same characteristics we see in a superhero. Why are we interested in Superman, or Batman, or Robin Hood, or any of the other "mythical" figures who appear to defy human limitations and do good for the common person and our world? I might be inclined to believe that we all secretly dream of being that way ourselves.
I cannot speak for other people, but what attracts me to the Sherlock Holmes stories is how smart and versatile Holmes is and how complicated the cases that he solves are.
In the Holmes stories, Holmes is always able to learn so much simply by thinking. He can look at a cane that someone has left at his apartment and know what kind of dog the man had, for example. I am also intrigued by how many kinds of things he knows about -- kinds of cigar ash, typefaces in newspapers, the KKK. Finally, I love how he can transform himself into things as different as a stable groom, a minister, and a opium addict.
In addition, the cases he solves always seem so complicated. Because of this, there is a lot of suspense and intrigue for the reader as Holmes figures out what is going on.