I've been pondering this for most of the day, with no definite answer springing to mind. I think that when I wrote the post about the Hunger Games, I was considering it as mild science fiction because you could read for a good while without realising that it was a science fiction book. So my thinking has been going in that direction - how long would it take a non-science fiction reader to realise that they were reading a science fiction book?
If it's set on a space station or off-world colony or spaceship the game is up straight away so on this basis I'd put people like Iain M Banks and Peter F Hamilton into the hardcore category. I think I'd like to add that some serious science or basis in scientific fact should be used for books to fall into the hardcore category. I like to be able to go and look things up and say 'oh, this is what that technology might develop from' (read Asimov's original Foundation trilogy and then Philip Ball's Critical Mass to get an idea of what I mean).
With Pern novels, that you mentioned in another thread, I was convinced I was reading fantasy for a long time, and by a long time I think probably more than one or two books. These are books about dragons, they can't be science fiction, must be fantasy. Fools the casual reader pretty easily and it's only once you finally get to the computer parts of the story that you realise it's been science fiction all along.
Then perhaps you go to read The Man in the High Castle and could be convinced this is a history book, it's just speculating on what might have happened had real events turned out differently. That's not science fiction surely, just speculative fiction, and isn't all fiction speculative?
Taking this a step further, Neal Stephenson's Quicksilver won the Arthur C Clarke award, which recognises outstanding science fiction, and that definitely feels like a historical novel. Stephenson apparently argues that its science fiction because it's fiction about science, but I'd certainly class this as mild science fiction. I could probably give it to my Nan to read and she wouldn't notice that I'd tricked her into reading a science fiction book. To be fair, I didn't feel like I was reading a science fiction book either and found it a bit hard-going (Anathem on the other hand I loved from beginning to end).
So, to try and reach a conclusion. I think that hard science fiction needs to have a strong basis in real science and is immediately recognisable as being a science fiction book. Mild science fiction may have a science fiction setting but it is either not immediately obvious or other genres have a strong role in the book so that it's not immediately apparent that the story is science fiction.
I'm sure that other people would consider things differently, but that's how I consider it, and certainly the way my thoughts were heading when I posted that the Hunger Games was mild science fiction.