Well, first of all, what is it? For many sf writers the term 'hard sf' is a little like that embarrassing tattoo you got in high school and just can't seem to live down now that you have a real job. Hard sf devotees tend to use the term in regrettably silly ways . . . like to talk about how 'hard' they are, and to weed out people they think aren't 'hard enough' to play in their league. In return, people who write the various subgenres of SF that don't generally include multipage explications of imaginary technology complete with equations tend to respond with equally absurd claims that there is no such thing as hard SF and the whole concept of hard SF is just the science fictional equivalent of a No Girls Allowed tree house. As a result, the hard sf label (actually coined in the 1950s by Astounding Stories editor P. Schuyler Miller) often evokes the worst excesses of 1950s Agenda SF (1) . . . stories where aliens were enemies, women were green, and white men saved the free world with science. All of which has nothing whatsoever to do with the nuanced and complex stories being written by today's best hard SF practitioners.