I used to read a lot of science fiction, though very little these days. As a teen, that was almost my entire literary diet. There are strong concepts and weak concepts, good writing style and bad writing style. More than anything, though, I found my enjoyment of sci fi to depend most upon the ?science? of the fiction ? not so much whether it was consistent with the universe as we understand it, but whether the science motivated the story. I rank science fiction in three tiers.
There's honest-to-god sci fi, the kind that really plays ?what if.? What if life evolved on a neutron star? (Forward, Dragon's Egg) What if the behavior of large populations were minutely predictable, though individuals were not? (Asimov, Foundation) What if we could dramatically increase human intelligence, but only for a brief period? (Keyes, ?Flowers for Algernon?) The whole purpose of these stories is to speculate, and then pursue the speculation to its logical conclusion. Sometimes, the story doesn't appear to be science fiction ? not a laboratory or bug-eyed monster in sight. Niven's ?Not Long Before the End? describes a sword-and-sorcery duel, but the real point is to ask ?What if magic were subject to the second law of thermodynamics?? You can generally count on finding something new in these stories, and an interesting tale woven around it. Even if the writing itself is second-rate, the ride is worth your while.
The second tier is what most people think of when they describe science fiction: space ships and aliens and rayguns and, more recently, cybernetic implants. But here, the science has nothing to do with the story; it just provides some flashy props. Sometimes it works, if well-executed; Star Wars is a thrill, even though the same story could be told with frigates and Caribbean natives and dueling pistols. (Actually, the equivalence of mythic story lines was an intentional point of the movie.) More often, though, you're just getting a Flash Gordon adventure using wild costumes to hide the fact that you've seen it all before.
And down at the bottom comes the stuff which isn't science fiction at all, but a half-baked excuse for some other genre. The original Star Trek series started using el cheapo excuses like this to put Kirk on way too many alternate earths when the writers started running dry on ideas. (The prohibition gangter planet, for example.) When warriors take their laser lances and ride their robot horses into battle, you're reading medieval fantasy (and pretty crappy fantasy, at that). When ?alien DNA? turns a human into a man-eating horror, do not be fooled by the words ?science fiction? on the paperback's spine, or the book's inclusion in the sci fi section of Barnes & Noble. When ?cosmic rays? turn astronauts into superheroes, their transformation is about as scientific as fairy godmothers.