Can the manticore bite through steel? Does the dragon’s fire melt stone? What happens when someone gets hit with those mage fireballs? Any piece of fiction that has fantastical, sci-fi, superhero, or any variation of those elements needs to have consistent internal logic. It’s the thing that keeps us, the audience, rooted in the world even though Strongman is swinging a bus like a baseball bat.
I see internal logic as the rules of your world. If you have a fireball flinging mage, then the damage of their fireballs should be consistent. That way the reader knows that when the fireball hits a wooden shield it’ll char it, but if it hits a steel shield it won’t do any damage. Things like that, that remain consistent throughout the story, are what help keep the reader immersed in the world. If the fireball doesn’t do anything to a wooden shield, but turns a steel one into a pile of molten goop, that takes me out of the story because it doesn’t seem to make sense without an added explanation. If later in the story the fireball turns a wooden beam to ash, then I’ll start to think the writer doesn’t have any internal rules that govern the mage’s fireballs, making it hard for me to get into the story because it feels like there’s no consistency.
I’m using things mostly associated with fantasy as examples, but this really applies to any kind of story. It can be extended to all kinds of things in a whole variety of genres, from character traits, to tools, sci-fi gadgets, or even laws and the consistency with which they’re enforced. When any kind of “rule” is introduced to your world, you should stick to the general parameters of that “rule” throughout the story.
Rules could be things like:
The fireball cannot melt metal
The comm-link requires wifi access
The yeti needs to eat, like, a *lot* of food daily
And so on. The rules can be loose, or more like guidelines that can be bent this way or that, but in general there should be some consistency to their implementation. Maybe the mage fireballs can only melt metal if their being cast by an elder mage. That would be a good way to bend the rule, or add an addendum to it.
And I prefer to know the limitations of these abilities or powers. It makes it easier to build the world around them, to know how they would act in most scenarios. It feels like you’ve laid out your tools, and now it’s just a matter of how you want to use them.
Edit (February 25, 2019): This series is about the things that I've learned, or am learning, about worldbuilding. It's me trying to level up my craft, and documenting the process. These posts represent my personal approach to worldbuilding, and the way I do it might not be right for you. I'm not an authority on writing, and so everything in these posts should be taken with not only a grain of salt, but a heaping bucket of saline.