Seeming effortlessness.

When an author has done all the hard homework of clarifying physics laws, determining the precise mechanics of spacecraft, calculating detailed demographic and topographic information of multiple worlds, and working out how various beings interact n such a universe, the end effect seems effortless.

As with all stories, a detailed back story and history, a clear plotline and sub-plots, and excellent descriptive and conversational writing are necessary as well. But the details matter. Even if I don't see them ... maybe especially if I don't see them. 

For these reasons, among others, Stephen R Donaldson and Kevin J Anderson are two of my favorite science fiction authors. Rich storytelling, multidimensional characters, and a universe that is able to explain itself when it needs to.