I can't really articulate how I define science fiction. Generally, I would say that it is a What If engine (or, if it's written by Grant Morrison, a WTF engine), where a mostly plausible change is made to the world as it should be so that it becomes the world as it might be.
Today, I mostly enjoy comics. The Eternals, Guardians of the Galaxy, and X-men are my favorites. I also highly recommend anything Green Lantern or Booster Gold. Warren Ellis's Supergod is weird and gross and makes me swoon.