First, to avoid confusion, I think we need to define our terms for the purposes of this post.
By “sci-fi,” I mean the more narrow catagory, not the entirety of speculative fiction. I would exclude “fantasy.” Yes, yes, the lines blur in some cases, but for the most part if you had to pick a shelf, you know which one a given book would go on. I’m going to exclude past time travel and alternate history, but allow future time travel. Alternate history is fun, it’s just another animal.
I don’t want to pretend that I can invent the ten commandments of sci-fi. I don’t like hard and fast genre rules. Inevitably, the moment someone comes up with one, you get a William Gibson inspiring a million imitators who break it. But let’s try for a general framework.
First, and perhaps most obviously, sci-fi should ask “what if?” (Incidentally, Marvel occasionally runs alternative Marvel history comics titled “What If ____,” and they are usually a lot of fun.) If it doesn’t involve a future or alternate world, it belongs on a different shelf. This is where sci-fi and fantasy run up against each other, and this quality could be a broad description of the Speculative Fiction supergenre (is that a word? it is now).
Science fiction focuses on the “what if?” Where fantasy and space opera are usually much more character-driven, sci-fi generally forces us to consider the consequences of our hypothetical.
“What if we created dinosaurs from mosquito-borne dinosaur blood?” Well, it would start with “ooh, ahh,” but later there’d be a shirtless Jeff Goldblum, and then the running, and the screaming.
“What if we could upload our minds into any available body?” You’d have faceless assassins working for deathless multi-trillionaires in a neon and chrome dystopia.
“What if we fought a war against aliens and had to use near light speed travel to get there?” Relativity is a bitch, my man, and the past is a foreign country. But men still shoot each other.
But what makes books like Jurassic Park, Altered Carbon, and The Forever War work? It’s not really the science. Let’s face it, all of them assume away some limitations, or get the science wrong/oversimplified, or both. It’s not the characters, either. Even in great works of sci-fi, character arc can be negligible (although it needn’t be, and I would always argue in favor of well developed, fleshed out characters). I would argue that what makes a work of sci-fi great is how well the author analyzes the consequences of the differences between his world and the real one.
Take The Forever War. First, if you haven’t read it, stop reading this and buy a copy. It’s that good. Despite writing a military sci-fi novel based on his Vietnam experiences, Haldeman doesn’t just address the first-order consequences of fighting a war that requires near light speed travel. He puts a great deal of thought into the cascading problems caused by centuries of warfare. Cloning, language drift, cultural differences between cohorts of soldiers who may be born to entirely different societies, colonization, maintaining friendships across relativistic aging differences, Haldeman convinces us that he has seen the future he portrays. His vision is complete and coherent, everything he describes seems plausible.
That coherence, the plausibility, is really what sells it. People, alone or in groups, act like real people. Governments behave like governments we have already experienced. Except when he is intentionally bending what is possible under the laws of physics, they apply as we know them today. Future technology has rules, and those rules seem similar to known physics, and those rules are obeyed.
Contrast with your typical young adult dystopian romp. Not to disparage the genre (ok, that’s a lie), but the dystopian societies portrayed frequently appear to have developed merely to inconvenience the heroine (almost always a heroine, these days). Their evil ways are never fully explained, their cruelty is not rationalized. Bad thing happened, and so now we have stormtroopers who oppress teenagers. Shrug. We have some fantastic technologies, they will make a limited appearance when necessary for plot advancement, and then be placed in a mysterious uninventing closet when their appearance might be inconvenient. Hope you connected with the protagonist, and picked the correct side of her love triangle for your team hashtag.
Again, as stated above, good sci-fi can have excellent character development. But it isn’t required. Salvor Hardin is no one’ favorite character (that I’m aware of, maybe you have exceptionally odd friends), but Foundation is surely on any serious short list of sci-fi masterpieces. It has become fashionable to tell people not to fall too much in love with world-building recently, but if you’re writing sci-fi, it is your keystone. Do it poorly, and your book could be so disastrous that Hollywood makes an overwrought four part blockbuster out of your trilogy.