The turning point for me was the acendance of Ursula Leguin, a stoned-out hippie chick from the Northwest, as a "MAJOR Science-Fiction writer". It seems to me that sword-and-sorcery and various other childish fantasy themes took over and swamped "real" science fiction about that time (mid-70's).
The list of really first-rate writers of science fiction is quite limited- I would include Arthur Clarke, Asimov, Heinlein (but not his post-"Stranger in a Strange Land" New Age nonsense!), Fred Pohl (of course!), William Tenn, Theodore Sturgeon, Fredric Brown, A.E. Van Vogt, and a handful of others. (Note that Ray Bradbury is NOT on my list...)
Exactly. Of course there's never been agreement on what "real" SF is. A lot of writers like de Camp and Kuttner could easily change hats from "hard" science to sword and sorcery at a penny a word (at 80 wpm) to put food on the table. And in the 40s and 50s a lot of fantasy posed as SF. I recall the endless debates about it in the letters columns.
I began to lose interest in magazine SF about the time Ben Bova became editor of "Analog" back in the late 60s. I became aware of a liberal agenda and didn't subscribe to the magazine again until Schmidt became editor sometime in the 70s. I guess I've always preferred "Analog's" hard science extrapolative approach. The two best stories I've ever read combining hard science and powerful human themes were "The Cold Equations" by Tom Godwin and "Flowers for Algernon" by Daniel Keyes. I think it's still possible to write stories of that kind but no one is doing it that I'm aware of.
Henry Kuttner and his wife C.L. Moore, who often collaborated, should never be left out of any list of SF "greats." Some of Kuttner's pen names were Lawrence O'Donnell, Kelvin Kent, Lewis Padgett, Keith Hammond, Hudson Hastings, Woodrow Wilson Smith and C.H. Liddell. He produced an enormous amount of high quality material before his untimely death at age 44 in 1958. I loved his robot stories with Gallegher, the alcoholic genius, who got blitzed at his "liquor organ" and invented crazy stuff that he couldn't remember and had to untangle the next morning while suffering from a terrible hangover. The robot was never any help. Classics.