I think the genius of Conan Doyle's Sherlock Holmes character is a bit like the genius of ketchup. It is not the distinct presence of any strong spice or any huge, idiosyncratic trait--it is a matter of perfect balance. Ketchup has this universal appeal and usefulness because salty and sweet and savory. It has a universality that can be adapted to a lot of different dishes.
Likewise, Sherlock Holmes is cerebral and a pugilist, a philosopher and pragmatist, a lover of classical music and the lowest kind of grand-standing humor. Sherlock Holmes himself is a master of disguise. Do you want to make an action bromance movie full of explosions: Sherlock Holmes. A gay porn flick: Sherlock Homes. A clever modern drama with a swishy asexual star: Sherlock Holmes. A cartoon about a heroic mouse? A time travel comedy? A surreal tale of horror porn? Sherlock Holmes.
You can add any twist, any spice, to make the dish you want. But the underlying appeal, the fast track to fond familiarity, as finally appreciated by the gourmet ketchup vendor, is that the basic condiment is forgiving, smooth, and balanced in a basically unpretentious way. It is for the reinterpretor, the reader or the fan to decide they way want to consume their Sherlock. And that is why he will never go away--and while endless spun and rerun, never improved upon.