What made the characterization of Sherlock Holmes so unique?
He was the first literary detective to use scientific and medical inquiry in his cases.
Sherlock Holmes is an archetype--a detective who is equal parts encyclopedic knowledge and pure deductive reasoning. His author, Arthur Conan Doyle, was a marginally-successful physician who originated the idea of using techniques of medical inquiry for criminal investigations; prior to this, real and literary policemen relied on eyewitness testimony and coerced confessions.
We don't really get inside Holmes's mind (Holmes only narrates one story, "The Adventure of the Lion's Mane"), and instead we rely on the narration of Dr. Watson, who is much closer to a reader-identification character than Holmes.