|The March 1939 issue in which Asimov's first published|
story, Marooned Off Vesta appeared, 77 years ago!
“Writer X spends most of his time describing how the machine would run, explaining the workings of an internal-combustion engine, painting a word-picture of the struggles of the inventor, who after numerous failures, comes up with a successful model. The climax of the yarn is the drama of the machine, chugging its way along at the gigantic speed of twenty miles an hour, possibly beating a horse and carriage which have been challenged to a race. This is gadget science fiction. […]
“Writer Y invents the automobile in a hurry, but now there is a gang of ruthless crooks intent on stealing this valuable invention. First they steal the inventor's beautiful daughter, whom they threaten with every dire eventuality but rape (in these adventure stories, girls exist to be rescued and have no other uses). The inventor's young assistant goes to the rescue. He can accomplish his purpose only by the use of the newly perfected automobile. He dashes into the desert at an unheard-of speed of twenty miles an hour to pick up the girl who otherwise would have died of thirst if he had relied on a horse, however rapid and sustained the horse's gallop. This is adventure science fiction. […]
“Writer Z has the automobile already perfected. A society exists in which it is already a problem. Because of the automobile, a gigantic oil industry has grown up, highways have been paved across the nation, America has become a land of travelers, cities have spread into the suburbs—and what do we do about automobile accidents? Men, women, and children are being killed by automobiles faster than by artillery shells or airplane bombs. What can be done? What is the solution? This is social science fiction.” (Asimov, "Social Science Fiction")
- Gadget story: describes how some hypothetical thing might work.
- Adventure story: describes how some hypothetical thing might be used to solve a problem.
- Social story: describes what society might look like when some hypothetical thing exists.
There are a couple of curious things about this position though, even ignoring the quibble (which I'm sure the Good Doctor would actually have agreed with) that it's a rare story indeed that neatly falls into just one of these categories.
One of the first thing that springs to mind is that, while Asimov doesn't obviously dismiss other kinds of fiction as unworthy, it's clear that in his opinion "social SF" is best placed to reach the "heights" of literary fiction.
While I'm right alongside Dr. Asimov in agreeing that social SF can be much harder to write, and certainly requires more writing skill than simply sketching out a neat idea in a flashy fiction wrapper, even in the era he was writing in and before it should have been obvious: the basic fact that writing a saleable story of the first two species doesn't require the type of literary skills Asimov advocated for in SF doesn't actually preclude their application.
I'm not of course suggesting that Isaac Asimov was ignorant of what came before him - on the contrary, looking at his early work in particular you can almost see the impact of previous writers in its inverse: Asimov's prose is consciously direct and unornamented, and while he develops over the course of his remarkably prolific career  even to the end his stories lack so much in the way of literary flourish that he has proven a slippery target for the ordinary literary critic. For many, this is a huge plus: in fact, the ruthlessly plot-and-idea oriented writing that is Asimov’s hallmark is one of the things that made him so popular.  I’m not even suggesting that Asimov was himself without literary skill – in fact, despite constantly breaking cardinal rules of fiction (minimize exposition, show don’t tell, write natural, believable dialogue, and others) he demonstrates that he’s capable of some amazing characterization at important points in his plots (look at Liar! for example, or Second Foundation), not to mention the skillful way in which he contrived the narrative of (especially) longer tales, nesting plots and snapping together timelines in fascinating non-linear ways. 
5. Though not entirely impossible: Professor Emeritus James Gunn (University of Kansas/English Literature) seems to have done quite well, for one.