What separates the superlatively great from the merely very good?

Frederick Nebel is like a great hamburger or pepperoni pizza; Dashiell Hammett is steak au poivre that melts in your mouth, followed by a fine aged tawny port. Raoul Whitfield is a Lexus, Raymond Chandler a Bugatti (though given Whitfield's output of aviation stories, maybe he's the Spirit of St. Louis and Chandler a Concorde). Hammett is Giotto and Chandler, Babe Ruth; George Harmon Coxe and Erle Stanley Gardner are—  But you get the idea.

I'm reading one of the superlatively great Black Mask writers, one who would be right up there if he'd written more, and one of the mere very goods. (The superlatively great is Hammett, the would-have-been is Paul Cain, and the very good is Frederick Nebel, in the form of Crimes of Richmond City, five loosely connected stories that appeared in Black Mask in 1928 and 1929.) 

The Nebel has great moments of tension and even psychological insight, and one of the great comic crime fiction foils in Kennedy, of the Free Press. It also has archaisms that induce a smile in today's readers:

"`The skunks!' exclaimed Kerr. `Can't we run the pups down?'"

or

"`Drive to that old brewery,' he clipped."

It won't do simply to chalk up the first example to its era's greater reticence than our own with respect to swearing. Chandler in The Big Sleep and Hammett in "The Girl With the Silver Eyes" devised entertaining, evocative ways to suggest swearing without the archaically comical touch of "The skunks!" Perhaps one definition of greatness in a writer is the ability to solve narrative problems in ways that would not occur to lesser authors, and to turn those problems to his or her advantage. So here is your philosophical question, readers: What distinguishes a great author from one who is merely good, even very good? Examples welcome.

(Granted Nebel was near the start of what would be a prolific career that lasted into the 1960s. He may have lacked the confidence or the juice to blaze creative trails early on. But Hammett was still in his twenties when "Arson Plus" appeared and barely 30 when he wrote "The Secret Emperor.")

© Peter Rozovsky 2014