Definitions are murky when it comes to literary genres, but when I think of space opera, I think of several key features:
- Interstellar travel via faster-than-light drives or wormholes.
- Far-future technology that is semi-magical (but will sometimes be explained by minimally-plausible handwavium).
- Multiple space-faring civilizations with divergent worldviews that interact with each other through trade, diplomacy, and military conflict.
- An arc of galactic history - either explicit or implied - that covers centuries -- if not eons.
So what, exactly, appeals to me in this genre? When I pick up a space opera, what are some of the things I look for?
Well, first of all, I'm going to echo others and say that a successful space opera absolutely needs to have that sensawunda. The galaxy in a space opera should be chock-a-block with mysterious ruins, fearsome "old ones," and natural dangers that will lead our protagonists down surprising roads of discovery and exploration. Or, to put it another way, a space opera should inspire humility. As a reader, I want to walk away feeling like I don't know everything -- like there are things in the universe undreamt of in my philosophy that I should approach with circumspection and even awe.
Secondly, I like high stakes. I want the characters in a space opera to fight for things that really matter. Existential "fate of the quadrant/galaxy" threats work very well for this criterion (and happen to be awesome) -- but freedom, love, and truth are also excellent motivators. Whatever you choose for the lodestone, it has to be something that will force the principals to become better people in its pursuit.
Third, I like characters with agency. They should be animated by the belief that their actions can make a difference in the universe -- even in the face of the wondrous, the frightening, and the seemingly unknowable. They should want to make things better and should seek to understand whatever they encounter. They don't always have to succeed, mind you; tragedy does have its place here -- but nihilism and passivity don't.
And there are some smaller things I like as well. For example, I have to admit that I enjoy political intrigue and/or philosophical rumination. This does not mean I appreciate a space opera's getting bogged down in today's fad-of-the-moment. It does, however, mean that a space opera shouldn't avoid the big questions; after all, where are the aforementioned high stakes if we aren't exploring the purpose of existence, say, or the nature of good and evil? Indeed, space opera provides a fantastic context in which to meditate on these issues at a far remove from our present-day concerns.
Also, while I don't think aliens are a necessity, I do gravitate to them quite a bit -- especially when it is clear that an author has thought them through and consequently has struck the perfect balance between stretching the reader's imagination and preserving readability.
Do I think realism is dumb when it comes to space opera? Well, it depends on what you mean by "realism." If by "realism" you mean a close resemblance to our own mundane lives, then yes, "realism" is dumb as hell. But I think internal sensibility and psychological verisimilitude are a must. For a space opera to work, we must be able to comprehend its rules and, even more importantly, relate to the choices made by the characters. Ultimately, the story a space opera tells should, like the myths of old, map onto the habits and yearnings of the human mind; this is genuine realism, and it leaves plenty of room for Barsoom and all the rest.