Science-fiction is about what human beings will become in the future, how they will deal with the challenges of new technology, new social organizations, new experiences, and even new definitions of what it means to be human. Although it's not about technology per se, technology plays a major role in being an underlying cause of the changes providing these challenges.
So, it's speculative.
Now, a hundred or so years ago, it was widely regarded that all the known laws of physics had been discovered, and that theoretical physics was a "completed" science, with nothing more to do other than cross a few t's and dot a few i's. It was also believed that if you travelled more than 15 miles per hour in an automobile you'd probably suffocate...
The science of "tomorrow" (whenever that is) is going to be radically different from the science of today. In fact, given Moore's Law, and the general acceleration of the rate of change in our culture, it's probably going to be more different than it's ever been before. So different, in fact, that our brains and imaginations right now likely can't even begin to imagine (or perhaps even understand) what shape that science will have - there'll be new "laws", new discoveries, and the agreed orthodoxy of today will likely be swept away in an explosion of discoveries that are allegedly "impossible" today. Give a neanderthal your iPad; that's a gap of 50,000 years. Now expect a similar gulf in understanding to open between the world of today and that of only, say, a couple of hundred years from now.
Enter "hard" science-fiction. This genre, as far as I can fathom, prides itself on not relying on any science we don't consider "possible" today. So, no teleportation devices, no antigrav, no faster-than-light travel, no telepathy, telekinesis, or other whole bunch of stuff which often enables science-fiction to truly take wing. Instead, reaction mass, spinning habitats, hydroponics decks as far as the eye can see; and lots of grim, grey vistas of grease, lube oil, and rusty chains dripping with water.
Here's my problem: what's the point? If you're writing speculative fiction, then *speculate*. Don't restrict yourself to our 21st century "angels-on-a-pinhead" orthodoxy: be brave, paint with massive brush-strokes. I'm not saying we should abandon all "logic": we're writing stories, after all, and they require a sense of verisimilitude if they're not to be rejected instinctively by a reader. But at the same time, it seems horribly sad to decry a work of fiction which breaks through the glass ceiling of modern "hard science" orthodoxy and dares to dream.
Sometimes it even seems to me that "hard science-fiction" isn't science-fiction at all, but a kind of technological fetishism, and obsession with the minutiae of gears and crankshafts at the expense of the flight of thought. In that sense, space opera remains the bearer of the flame; somewhere speculative fiction is still permitted to truly speculate, without the artificial constraints of today's priests of orthodoxy.
There. That's my beef. I love me some science-fiction, right to my core. And I hate me the niggardly people who hounded Galileo to the pyre. I want my science-fiction to soar like Icarus and make me think something I've never thought before, not remind me that I need to clear the garage and do my taxes.
And if our wings melt while doing so, well, at least we flew ;)