A critiquer once took me to task over whether Ghosts of Innocence could really be classed as science fiction. To count as such, the accepted wisdom is that science in some form or another should be at the root of the story. Take away the scientific premise and the story collapses.
I certainly think Tiamat's Nest fulfils this requirement. A secretive intelligence emerges spontaneously in the heart of the global network and manipulates people and the global economy to its own ends. The story hinges on science-based premises involving a mix of advanced computing and emergent phenomena.
Ghosts, by comparison, is firmly at the soft space opera end of the spectrum. Yes, there's space travel and lots of cool toys, but the story is largely about the human motivation of revenge. It could equally well be transposed to many historical settings, as long as one side possesses a significantly destructive weapon to provide the triggering event, and which could be subverted and turned against its owner.
The plot may not be hard-science driven, but as long as sufficiently plausible scientific principles drive the technology in use then I think it's fair game. It's speculative fiction, and I'd be hard-pressed to say what else it could be described as.
Other stories take a range of liberties with science in pursuit of a gripping tale, particularly amongst recent popular movies.
Star Wars has the Force, which brings in a touch of mysticism. But while the Force is not really explained, it is never viewed by practitioners as being anything magical or supernatural. Rather, it is seen as a part of the natural world, which is absolutely the domain of science.
Avatar came close to breaking the "rules" for me with unobtanium. This levitating mineral was just dropped in with no clear rationalization, and as a viewer my science-bullshit meter was twitching although I pushed it to the back of my mind because I was enjoying the story. I have since learned that the backstory talks about a superconducting mineral with extremely powerful and unusual magnetic properties. The explanation may be tenuous, but for me it pays enough lip service to known physics to restore a measure of credibility.
I think this also illustrates an important point in worldbuilding. Not everything needs to be spelled out to the reader (or viewer) but it helps if the rationalization is there in the mind of the author.
I find something like X-Men harder to swallow. The idea that genetic tricks can magically confer scientifically doubtful superpowers is never explained beyond hand-waving and an appeal to the emotive term "mutations". Although the story has some scientific window-dressing, I'm more inclined to class this as fantasy.
As you see, there is a broad spectrum where sci-fi drifts into fantasy, with no clear boundary between the two. For me, a lot depends on how seriously the worldbuilding treats the science. As such, I'm happy to class Ghosts as sci-fi.
Having said all that, how much does it matter? A story is a story. If it engages and entertains, then how important are pigeonholes?