"SF is SF because the author consciously or instinctively believes that the universe runs by predictable and knowable laws. In addition, the author's world, while radically different from our own, is achieved by applying some sort of transformation on our world as the author knows it. The fantasy is created from whole cloth, the author's world is just plain different and goes from there. In SF, there's the unspoken premise that, given the author's assumptions, we could have gotten from here to there. This rational chain of logic is what makes a story feel like SF, even if it has elves and vampires in it."
It's an interesting take, with a lot of promise and some interesting implications. Of course, the quality of the science fiction becomes directly proportional to the quality and plausibility of the initial assumptions and the logic used to derive the SF world from those assumptions. This definition squarely places many alternative history novels in SF, particularly one with time-travel elemenst (like, say, Turtledove's Guns of the South). Future history, even in the near term, also would fall into this arena in most cases, particularly where science is a driver for the plot. This places many disaster movies clearly into SF, especially the current trend of future climactic catastrophe books and flicks.
This definition does, however, have the effect of excluding many genre-straddling works outside science fiction. Buffy the Vampire Slayer had many SF elements to it, but its clearly horror/fantasy by this standard. It also means that Star Wars probably isn't SF, as no amount of pseudo-scientific arm-twisting could get us from there to here. The Force alone precludes that, no matter how many midichlorian-inspired retcons Lucas foists upon us. Star Wars is high fantasy by this standard. And I'm OK with that.