All the recent discussion of design in the Star Wars universe has led to a fair question of how deeply someone designing a universe and the things in it has to go to make the thing plausible enough for its task — which, in my opinion, is to keep the audience engaged all the way through the work without once saying, “now, wait just a minute…”
Other worldbuilders will have to answer this one when talking about their own works, but as for me, in general, I try to build my worlds at least two questions deep — that is, you make your creations robust enough to stand up to a general question and then a more specific followup question. Thus:
Reader: Why did you give your genetically engineered soldiers cat’s eyes in Old Man’s War?
Me: Well, relating specifically to pupils, it allows better filtering for the range of visible light the soldiers work in across different planets and environments.
Reader: Okay, but why not just engineer eyeballs to make smaller round pupils?
Me: The scientists in the OMW universe find it easier to work with pre-existing genetic code than develop new code, so they do that whenever possible.
And for about 90% of your readers, that’s going to be sufficient rationale. For about 10% of your readers, it won’t be, but at some point, and simply as a practical matter, you realize that some folks aren’t going to be happy with your worldbuilding no matter how far you drill down, and that you can just sort of accept that as the cost of doing business in a geek-rich field like science fiction. To a very real extent, what you’re aiming for is sufficiency, not completeness.
(Mind you, that’s if you’re creating the way most people do, which is to have the world come out of the story, not the other way around. Tolkien, as an example and if memory serves, did it the other way around, which is that he built the world in detail first, and then told a story (two, actually) inside of it. You certainly can do it that way, and it is frankly awesome when pulled off well. But also it’s sort of the long way around, and recommended primarily for nerdy, vaguely OCD people with secure day jobs and lots and lots of time to kill.)
I think by and large the OMW universe functions at the “two questions deep” level, although I suppose it does depend on which two questions someone asks. To be sure, I know of at least a couple of places where the universe is barely a single question deep, which was bad worldbuilding on my part, and the only thing to do about it at this point is not call attention to those specific places. Please move on, nothing to see here [insert Jedi handwave]. But overall, it’s robust enough (and written well enough, which is a critical point) to get most people through each book without stopping to ask questions about the details therein.
Which is what I want, personally: If you get through the work before you start nitpicking, that qualifies as a victory condition for me.