Annalee Newnitz has an article on io9.com about the "ten major themes and ideas shared by HF and SF." Go take a look.
All then, I think, would fit comfortably under an uber-theme--a fascination with systems, both human and cultural. That is, how things work, and how they play out. Fiction is the great play-test of our imagination: we ask What If...? and let the experiment run. Both HF and SF run the experiment rigorously (the sub-genres not always; their concerns are different). The longer the fiction, the more numerous the parameters.
But, oh, for writers and readers of a certain bent, it's enormous fun. I think that when those of you who have read Ammonite or Slow River read Hild, you'll see that in many ways the concerns of Hild are those of my science fiction novels: how things work, and how my character/s influence the world, or at least their part of it.
Oh, I am so eager to share this book with you! I've no idea how I'll keep my mouth and blog strings shut. Just have to drink more wine and distract myself with luscious women who bake...