Noted: from Samuel R. Delany's "Dichtung und Science Fiction"

Also worthwhile: replacing "theme" with "trope" in this passage.

[T]he truth is...you will not find the key to science fiction in a survey of scientific, or even science fictional, themes any more than you will find the key to poetry in a survey of romantic, or poetic, themes. At best such a survey is a pretext for exposing new readers to a range of texts which will begin to familiarize them with the field's conventions, language, and semantic formalities. But to the extent that the pursuit of themes becomes a serious scholarly endeavor whose envisioned end is some goal of thoroughness, comprehensiveness, exhaustiveness, and critical mastery, the results will be more and more impoverished, the fruits more and more dessicated--less and less nourishing to the critical hunger for insight, resonance, and understanding. Themes only provide an intuitive, uncritical similarity within which true distinctions may be teased apart. But a theme itself has the same mental structure as a prejudice and must be treated, critically, with the same skepticism. Thematics--at least as we now know them--are useless for gaining any sophisticated insight into science fiction for the same reason thematics are inadequate to reveal the workings of poetics. For just as poetry may be about anything, in any number of ways, including science fiction, science fiction may be about anything, in any number of ways, including poetry.

Science fiction is no more a collection of themes than it is a collection of rhetorical devices. It is much better seen as a tension between subject and object it teaches us first to be sensitive to, then to expect--an expectation which it proceeds to exploit in as many different ways as there are different SF texts. It is a set of questions we expect to be answered about the relation of word and world, character and concept, fictive world and given world; and any given SF text can foil or fulfill those expectations in an infinite number of ways to produce exciting science fiction.

(To clarify: my feeling is that at this point the major sf discourse seldom even reaches the "inadequate" level of thematics because it is so busy concerning itself with "tropes", at least one step down the ladder of adequacy, instead.)