In my callow youth, when I first started trying to write SF&F fiction I thought the most important part of a story was the idea - the original shattering insight around which you can hang the story. At that time I was paranoid about the ideas that came to me and ground my teeth every time I read a story that seemed to steal one or more of those precious insights. I even wrote a story, lit by the fuse of bitter resentment, about someone who is the fount of everything that happens and only he knows it (can anyone say "hubris"?). My fear was that all the big ideas I had would be played out by the time I got round to writing them up and leave me with nothing to say.
Now I'm older, though still very much starting, and I realise that originality is not everything. Other factors, such as character and plot (though like SJD I'm leery of that word) matter more. A coherent story needs much more going on than a gosh, wow moment. I've often read in writing guides that good writing can make a bad idea into a decent story and many editors claim they would take a good story over a good idea any day.
I've also learned that the stories in the short fiction magazines now may have been submitted long, long ago. Stories can take months to be rejected and, paradoxically, even longer to be accepted. For that reason, if no other, true originality is rare.
I guess the other reason it is rare is because relatively few SF writers are tuned in to technology enough to suffer those moments of clarity. Originality is rare because we are all half-informed about what is coming down the pike.
It is also something of a blessing that originality is so precious because if the short fiction mags only printed what they had never seen before they would be pretty thin - though very entertaining.
Finally, and this is observation is perhaps triggered by that smoking fuse, I've read lots of stories by well-established names which build stories around very hoary ideas. Stories that would get you roundly booed if you submitted them to a writer's workshop. Maybe I'll know I've arrived if I ever do that and someone calls it a "brave re-imagining of the genre's most treasured tropes" rather than a sorry re-tread of a much over-worked idea.