I think there are really two competing definitions of science fiction, and that’s the genesis of the question in the OP.
First, there’s the concept of science fiction as being stories that revolve around science, technology and its applicaitions. This can be through adding new tech to existing society, or merely by transporting the reader into a different or future world with new technology. All too often, these stories focus on the gee-whiz factor and the setting, and less on the characters and their conflicts.
The second example is the speculative or extrapolative fiction that RickJay describes. This doesn’t necessarily have to involve technology at all; in my science fiction literature course in college, one of the stories we read was “The Country of the Blind” by HG Wells. This story definitely fits into the speculative sci-fi mold, although there is NO advanced technology or science involved.
The better science fiction combines the two definitions, and uses the science or technology as a sort of “lever” to get the reader to consider some alternative point of view, or point out consequences, etc… The stories however, are fundamentally about people, not technology. As much as I didn’t really like “Left Hand of Darkness”, it’s a great example of this- it’s fundamentally a story about people and characters, and the technology/different humans are vehicles for setting up the story, not the point of the story itself.
So to the point that harder science in sci-fi advances the plot better, I’m all for it. But concentrating on having exact orbital mechanics, or in having totally consistent speculative technology to the detriment of the actual tale being told and the point the author is trying to make you think about, is a bad thing in almost every case.