[whswhs]

My primary take on this is that a simple dichotomy—space opera is X, hard sf is not-X—can't possibly be right, because there's a lot of sf that's neither space opera nor hard sf.

Space opera has an attractor: The use of interplanetary or interstellar space travel as a focus for larger-than-life action/adventure. It's cinematic in the GURPS sense.

Hard sf has an attractor: The serious exploration of speculative scientific ideas through drawing out their implications on the basis of actual theoretical and empirical science.

There are other attractors. There is, for example, the expression of social commentary, criticism, and satire through the safe medium of portrayal of a fictitious society somewhere else, in the style of Swift, Lem, or Pohl and Kornbluth. Or there's worldbuilding for its own sake; this can be hard sf, as when Poul Anderson or Vernor Vinge does it, but it can also be more about making up exotic cultures, as when Gordon R. Dickson or Cordwainer Smith does it. There's even an entire genre that has the same cinematic appeal as space opera, but that minimizes or avoids space travel: Planetary romance in the style of Burrough, Moore, or de Camp.

Bill Stoddard