One of the more important aspects of writing the fiction of the fantastic is the creation of internally consistent rules of magic. Whether the magic is of the sorcerous variety or the technological doesn't matter for this formulation. As mentioned before this has to do with the double charge against the reader's willing suspension of disbelief due to the combination of the narrative being fictional and the world being one that is made up or extrapolated. For the reader to have the buy-in necessary to a successful reading experience they need to feel that the logic of the world is compelling.
The funny thing about this is that another principal of successful story telling is rule breaking. This isn't always the case, but in much of heroic or romantic fiction, the protagonist is going to end up having to overcome some sort of impossible odds. Whether that triumph is against an unjust system, a BBE (Big Bad Evil) type villain, the obstacle to romance, or against his or her own inner demons doesn't matter so much as the simple idea of overcoming. In order for that overcoming to be satisfying to the reader, they have to believe the seemingly mutually inconsistent ideas that the protagonist has no realistic chance at success and, at the very same time, that the protagonist is going to triumph and good will win in the end.
A significant portion of the fun of reading an F&SF narrative is trying to figure out how that triumph is going to occur. In the best stories, not only is the reader surprised by the eventual resolution, but they are also able to look back from the end of the story and realize that the ending was completely set up by the preceding events and in some ways almost inevitable. The best way for this to happen is for the author to set up the rules so that they are consistent and that a consistent application of them will doom the protagonist while simultaneously structuring them so that there is a loophole or some way to break the rules that is consistent with them that allows the protagonist to emerge triumphant at the end.
So, you've got to have consistent, believable rules. They have to apply throughout. They have to be constructed so that there is some way to break or avoid them that is also believable. Simple, no?