I think there are five things I look for in an SF work:
Ideas
Plot
Characters
Storytelling
Prose
The quality and originality of the speculative IDEAS is very important when it comes to speculative fiction. Truly original ideas are generally better than simply giving old ideas new twists or exploring them in new ways, and that’s definitely preferable to simply rehashing old tropes in predictable ways (at some point that no longer truly counts as an ‘idea’ at all). The consequences of these ideas should be extrapolated and explored in as much depth as possible. And a bunch of different ideas all being played with at once is almost always greater than the sum of its parts.
A story’s PLOT should be exciting, intriguing, compelling, and/or powerful. It should put you on the edge of your seat, keep you pondering the possibilities, force you to turn the pages, and leave a deep impression upon you when all is said and done. It should never be contrived, forced, or (worst of all) boring.
The CHARACTERS should be well-drawn, believable, and distinct. If at least some among them are meaningfully changed during the course of the story, that’s all for the best. Characters should never be flat, forced, or dull.
The STORYTELLING needs to be clear, concise, and effective. Pick any two. Although having all three isn’t a bad thing, either. At its best, storytelling will enhance the characters and plot and ideas. At its worst, storytelling will interfere with the characters and plot and ideas.
An author’s PROSE should be beautiful, evocative, clear, and concise — or, at the very least, some effective combination thereof.
Confused by some of the distinctions of the five facets? The plot is what happens. The characters are the people to which it happens or who make it happen. The storytelling is the actual means by which the author communicates his plot and his characters to the audience (his choice of scenes, his narrative structure, his choice of where to begin and where to end). The prose is the actual language upon the page. The ideas permeate everything — although the speculative ideas which are the bedrock of speculative fiction are generally found exclusively in the plot and characters.
In truth, I think these five facets are important in any work of literature. Perhaps ideas do not need to be speculative in other genres, but I still think a work will benefit from charting a new course or playing with an original concept, rather than simply rehashing old tropes. For example, look at Sherlock Holmes or Harriet the Spy.
In the end, an effective book doesn’t necessarily need to excel in every facet. Rendezvous with Rama, for example, is one of my favorite books — despite the fact that its characters are just a few degrees away from inhabiting Flatland. The book succeeds because it is driven by a compelling plot and some ideas of epic scope, with transparent storytelling and prose that don’t get in the way. (And, actually, that last is not true: The storytelling is actually brilliant, albeit subtle. One of Clarke’s storytelling decisions is, in fact, responsible for Rendezvous with Rama being a classic rather than just a pretty good book. I am, of course, talking about the last line.)
