Even though I've started the complete Holmes several times, I've never actually made it all the way through. And the more I read, the more obvious why that is: Doyle really didn't have enough material to fill four novels and fifty-six short stories' worth of paper. The plots are fairly trite, the mysteries are sometimes clever but mostly commonplace, the insights into human nature are fairly shallow, and the prose is expedient if unremarkable. (The pacing is good, I'll give Doyle that.)
But there is one thing Doyle had that makes up for all the other shortcomings: he had a frickin' incredible character in Sherlock Holmes himself.
Even 120 years later, the dude is sui generis. He's awesome. Holmes's contention that you can learn everything there is to know about someone by studying the smallest item in their possession -- as he does in "The Adventure of the Blue Carbuncle" by reconstructing a stranger's history and identity by examining his hat -- is endlessly fascinating. So too his axiom that if you can eliminate the impossible, whatever remains, however improbable, must be the truth.
It's a great reminder that, in mysteries as in science fiction as in fantasy as in any other genre, good characters are the most important ingredient to the story. Doyle has proven that all it takes is one.
(Oh, come on, Dr. John Watson isn't really a character. He has one of the most commonplace names imaginable, he has hardly any distinguishing characteristics besides his war wound -- which moves from place to place depending on the story -- and his dialogue consists chiefly of such insightful statements as "My dear Holmes, you must be jesting!" and "Really, Holmes, I don't see how you could have possibly deduced that." The dude's a foil for Holmes and a surrogate for the reader, plain and simple.)
I find that when I think back on the great stories I've read in my lifetime, SF/F or otherwise, it's generally the characters that I remember. That's why I can barely remember a single plot from the original Star Trek, but I know the triad of
Ideally, in a great story you have a terrific plot that dovetails with the characters. (See Macbeth and Hamlet.) That's the whole point of a plot in a first place: to test characters' beliefs and abilities, their credos and promises and premises.
That's not to say that you can't have a great story without great characters. I've read most of the great SF novels of Doyle's Brit contemporary H.G. Wells, for instance, but the only memorable character I can recall from the canon is Dr. Moreau. The Time Machine might be one of my all-time favorite novels, but the only character from that book I can remember with any degree of clarity is Weena, the little Eloi woman who gives the Time Traveler a flower. Still, even though The Time Machine is a greater story than anything Sir Arthur Conan Doyle put to print, I'll bet Sherlock Holmes stays in the popular consciousness longer.
Update 2/19/09 @ 12:39 AM: Fixed the trio of Star Trek characters to Kirk, Spock and Bones. Duh, I knew that.