Let's continue yesterday's discussion. Just what is hard SF?
Hard science fiction is a story where some aspect of science or technology, usually speculative, is so critical that the story collapses when it is removed. Imagine Arthur C. Clarke's Rendezvous with Rama without Rama. No story. (Side note: the linked Wikipedia article claims Morgan Freeman is planning to make a movie of this story, to be released in 2009.)
Clarke's novel is also a good example of a mileau story. I won't summarize it here because the Wiki article does a fair job. The story is about Rama.
Hard science fiction is not for everyone, but those who like it really eat it up. I believe most readers who would enjoy hard science fiction are reading its close cousin, the techno-thriller. The techno-thriller reader thrives on information and wants to know about all the details. They want to know how the submarine works. They want to know how the missile was launched.
Hard science fiction has a lot in common with the techno-thrillser. Hard science fiction readers want to know how the spacecraft propulsion works. They want to know about the orbital mechanics of that trinary star system.
If hard SF has so much in common with the techno-thriller, why is hard SF a dying art? I've heard the arguement that science is getting too complicated for not only the readers, but also the writers. Balderdash. There are plenty of writers who understand what's happening in research today because some hard SF writers participate in that research as their day job.
No, I think the reason is more superficial. Sci-Fi, which I consider a derogatory term for most Hollywood productions that pretend to be science fiction. (In some circles, it's considered an insult to refer to serious science fiction as "sci-fi." Much preferred is SF, though the bookstore employees might not understand what you mean. Sad.)
Hollywood "sci-fi" is generally not considered serious unless you refer to how much they take to the bank. There are exceptions. Clarke's 2001: A Space Odyssey comes to mind, as does A Clockwork Orange. The vast majority are either dreary bores or simply flashing lights (to borrow a term from Harlan Ellison).
Most of the techno-thriller readers I know are the kind of people who wouldn't be caught dead watching Battlestar Galactica, especially the original. Put a copy of Ben Bova's Mars in their hands and I think they will discover they really enjoy the sub-genre of hard SF. I've seen it happen before. The problem is, these readers think of R2-D2 and spaceships that roar in a vacuum, and that turns them off when you recommend a good hard SF book to them. If they can get past the stigma of sci-fi, most would really enjoy hard SF.