Not all fiction is created equal.
To create any good piece of fiction, you have to get a lot of things right: prose, dialogue, pacing, character development, story arc... The list goes on and on. But to create good speculative fiction -- meaning sci-fi or fantasy -- you have to build an excellent world to hold your story. And that's no small task. I should know; for my college thesis as a folklore major, I wrote a 250-page science fiction novel.
I've been hearing, reading, and watching speculative fiction basically since I was born. (Actually, if you count all of the books my mom read out loud while she was pregnant, I've been absorbing spec fic since I was in utero.) Obviously some pieces were better than others, but there were times when the writing was good, the characters were compelling, and the story was full of all the right action, but something still rubbed me the wrong way.
It took me a while to realize that that was because the worldbuilding was off. And after studying some of the most successful worldbuilders, taking a worldbuilding class, and building some worlds of my own, I've come up with a list of worldbuilding rules*. I hope you find them helpful and enlightening.
1. Establish the laws of your world. The laws are what make the world you're creating different from our own. Make sure you understand how your world works, what is and isn't possible there. Anticipate questions your readers might have about some of your word's fundamental mechanics, and work the answers into your story. And if you set up the laws well enough, they might help you make tough choices without resorting to frustratingly plot-driven devices.
2. Follow the laws you've established (this should really go without saying). Have you ever played that game where you try to find inconsistencies in movies? Maybe an actor's hair shifts positions within a scene or more food suddenly appears on a character's plate. You don't want people to play that same game with your story.
3. Don't break your audience's trust. There are so many ways to do this. You could fail to follow the laws of your world, you could underestimate your audience's intelligence, your story could involve some superfluous elements. My personal least favorite is when worldbuilders blindside their audience by revealing a key piece of information at the very end of a story without any foreshadowing or allusion leading up to the conclusion. Your audience expects to be treated like intelligent consumers of speculative goods, so treat them as such.
4. Leave some mystery. There's a fine line between a satisfyingly informative conclusion and annoyingly being told all the answers. If you leave some things unsaid, you force your audience to engage with your story even after they've technically finished it. Just be careful that the mysteries you leave aren't actually oversights in the laws of your world. A good rule of thumb is that if it has to do with the mechanics or foundation of your world, it needs to be addressed in the laws.
* This isn't an exhaustive list, and I will continue to add to it as I learn and experience more worlds.