Internal Consistency-- the unbreakable rule in science fiction and fantasy
I like to find ways to circumvent the rules of storytelling and often enjoy it when others do so as well. The rules of writing keep things consistant, but another word for consistent is predictable. But while most rules can and should be broken from time to time when it makes a story better there is one type of rule that can never be broken in a good story. That rule is the one that you create for the story.
Internal consistency is important in all fiction, but it becomes doubly important when you are dealing with speculitive fiction with internal rules that often break the rules of the world as we know it. But while it is important in worlds of high magic and fantasy they are important in all stories. You can't write a romance set in the 18th century and then have one of the characters call for help on a cell phone.
An example of a movie that fails in its internal consistency, at least in my eyes is "Next". The main character can see two minutes into the future, except when it is directly connected to a specific woman then he can see farther. But as the movie nears it's conclusion the rules are stretched and then ignored. And while the writers were aware of the problem for me they weren't successful.
For science fiction I often find a problem that I like to call collective amnesia syndrome. There is sometechnology, superpower, or other element that is in the show. Perhaps a time machine, or a character who can heal people with her blood, but while they may use that ability occationally if i isn't useful to the plot they ignore it entirely. I mean how can you find a time machine in one episode of a show and he next week not point out that it could easily solve the problem they are facing. I understand you can't expect the writers to always remember everything, and some story elements are so powerful they become a problem, but good writing will explain why it doesn’t work rather than simply pretending it doesn’t exist?
When writing you can trust that a reader will in general accept any stretch of logic so long as it is written into the basic premise, but nothing will turn off readers and viewers of a show faster than the writers ignoring the rules of the universe they created. So while the other rules of writing can be ignored, the rules you create for your story have to be sacrosanct.