Science fiction is a three-headed beast, like the one that guards the gates of Hell. Every good sf novel has three main characters. First come the actual characters. Second, the science ideas. Third, the universe in which both characters and ideas operate.Read the whole thing. I'm going to fiddle a bit with Moriart's "coherent unity" triangle (not because he's necessarily wrong, but because it's just so much fun) and add the corner of plot. After all, if the characters don't really do anything then few folks will keep flipping pages. And it strikes me that broadening an already existing element to include thematic emphases (of which 'science ideas" are just a specific application) gives us not only a formula for excellent SF, but also for good books in general, the kind you can plow through in a single sitting and that stick with you for the rest of your life.
Cut through the eternal debate about whether this or that book is "hard sf" or "literary sf" or "space opera" and you can make a pretty good argument that every book in the genre can be mapped onto a triangle composed of those three cardinal points: character, science, worldbuilding. A good sf novel keeps you reading because you want to find out what happens to the characters. Or because you want to understand the science. Or for the sheer thrill of exploring the imaginary world the writer has invented. If you don't keep reading for one of these three reasons, then either it's not an sf book ... or it's not an sf book you're going to finish.
Of course the best sf keeps you reading for all three reasons. It defies categorization by melding characters, science, and world into a brilliantly coherent unity. That's what we're all secretly hoping to find when we open a new sf novel. We don't find it often, because even the very best writers can't hit a grand slam every time they come to the plate.
(Picture: CC 2010 by gfpeck)