There is one thing, though, about this genre or sub-genre—
Why is it that all too often, space opera features a back-story where humanity has returned to having an overall, ruling royalty—
Take a step back for a moment, and consider the evolution of this whole "royalty" thing. Early tribal and family groups had leaders, usually being the strongest, fittest and best suited to both survive and to lead. We can observe that today in the animal kingdom at large. The alpha male of a kangaroo flock near where I live is a cantankerous old sod that carries plenty of scars from fending off pretenders to his "throne".
As humanity became more "civilized", hereditary kingship overtook the older concept of tribal leaders being those best suited to lead. Sure, there is nothing stopping the incumbent's genes producing a "good" ruler, but it sure as heck doesn't ensure production of the "fittest" or "best". Consider the state of the European royalty by the late nineteenth century. Most of them were related. Queen Victoria was cousin, mother, grandmother or aunt to most of the European royal pack! So the inevitable happened—
At the same time that royalty were doing their best to interbreed themselves out of existence, growing social awareness led to increasing upheaval against the hereditary rule concept. Consider for example the Stuarts' adamant view of their divine right to rule, which was a major contributing factor to the English Civil War that saw Charles I deposed and beheaded. The state of play today is that the concept of the all-powerful ruling royalty has all but disappeared. The few remainders, certainly in Europe, are only figureheads and supposed tourist attractions (not to mention grist for the gossip magazines).
From at least as far back as Asimov's wonderful Foundation series, space opera all too often expects the reader to believe that humanity suddenly returned to the concept of a ruling, all-powerful royalty. Walter H. Hunt's Dark Wing series, a piece of militarist space opera I otherwise quite enjoyed, had a back-story of Earth having suffered greatly during a War of Succession. I can accept the concept of wars being fought between different factions or creeds seeking to rule, but wars being fought to put an all-powerful ruling royalty back in charge? The internal critic just leapt into warp drive.
I can readily suspend disbelief about all sorts of technologies and alien life forms, but the concept of an inexplicable and sudden urge to support a royalty rerun? It does help, however, if there is something particularly interesting about the ruling party that adds to the story. For example, Simon R. Green's Empress Lionstone in his Deathstalker series was such a psychotic, homicidal and insanely nasty bitch that I just had to keep reading. Although, after the inevitable rebellion was won and the Empress deposed, I never quite understood the populace's sudden desire for a constitutional monarchy. Emperor Palatine in George Lucas's Star Wars was another that was a little different—
Chris Bunch and Allen Cole with their Sten series had an Emperor voted into the role of all-powerful ruler-for-life because he controlled the energy source which made interstellar flight possible. As much as I enjoyed Sten, that concept was a bit hard for me to swallow. Of course, the interesting twist was that this Emperor had made arrangements which ensured that clones of him automatically appeared after he had died, following a suitable period of the populace experiencing life without the energy source that he alone controlled. The rule-for-life became a rule-for-many-lives-of-one-man with the populace apparently generally accepting his regular reappearance.
In fairness, probably every other reasonably expected alternative has been explored and used, but we seem to keep returning all too often to the same gambit. We are also often asked to accept that the all-powerful hereditary ruling elite are usually the product of a long-lived, unbroken hereditary line. Green's Empress was the latest in a 900-year dynasty. The wonderful Frank Herbert in Dune had Emperor Shaddam as the latest in the 10,000-year House Corrino dynasty. Asimov's ruling royalty in Foundation, prior to the collapse of the Empire, was another product of extremely long-lived dynasties.
Why such long dynasties? Are we expected to accept that humanity, a species that seems to become more skeptical of things as it ages, accepts that situation unquestioningly? Or that in the high-tech far futures of space opera, other prospective claimants to the throne or anti-royalist rebels are never successful, or at least not until our heroes of that particular story arrive on the scene?
The thing is that it doesn't have to be like that. Kevin J. Anderson in his Seven Suns saga gave us an earthly ruling royalty with a public façade of appearing to rule and make the decisions. His Majesty was, however, just a figurehead behind which a faceless bureaucracy ran things, largely through the hands of one individual. When a king had outlived his usefulness, the incumbent, in at least one instance, was given a new identity and permitted to disappear into peaceful oblivion while the public mourned his supposed death. The heir to the throne had been carefully selected and prepared beforehand. No matter if the king had failed to produce a suitable heir—
So space opera rules, even if I don't necessarily like whom they have doing the ruling. With so many other angles, however, do we really have to keep resorting to the worn-out royalty concept?