To me, rules and realism are some of the most important things that I focus on. Particularly the rules, because the realism aspect usually grows organically from the obedience to the rules.
Without the rules, my fantasy world doesn't make sense. I have to work out why things are possible, make sure that the reader understands and make sure that the rule is carried to its full extent. I.E. Say I had fairies who were vegetarian. The rule is then obviously NO MEAT. Good. One thing done. We also now have a glimpse into the culture.
But now, this begs the question... Do they hunt? Instinctively, my answer will be no... After all, if they're big on vegetarianism, I don't think they'll want to wear fur. Nor do I think they will want to use the fats or bones for anything either. In fact... I think they'd see any part of a dead animal as an abomination. (Maybe vegetarians don't. Have no idea. I'm just rolling with the fairy culture thing.) That already opens up a myriad of other questions.
Does their taboo about killing things extend to warfare? Will that make them pacifists? If they are, do they have defenses? Can they in fact be pushed to fight back? How far must they be pushed?
All those considerations just from one rule. And if I get those right, I've taken another step towards realism. Easy, right?
Not always. Sometimes, there are more subtle cultural norms that are in fact norms, but that might not be hard and fast. Say... equality. Women might be considered equal... to men in their castes. So yes, a culture could consider itself to be egalitarian while they are still just as obsessed with ranks. But what would that mean? Oh... perhaps richer/more powerful girls get to have an education. Perhaps they get to fight in the army. Perhaps there isn't such a culture of chivalry. On the other hand, the ladies might get a larger measure of respect, because they're not just seen as baby breeders.
It all depends on other things. History, for one thing. Other rules, for another. Some rules overlap to cause a different outcome to the more obvious one.
That's why carrying through the rules are so very important. Because if the rules aren't carried far enough, you might miss a point where they overlap.
The reader might not.
And that will severely limit the realism in the story.
So... how do you approach rules in your writing? Want to write down some interesting examples in your writing and the effect they have? Or point out some things I missed in my examples? I would love to get a glimpse into the way you think about things. Not to mention that it's fun. ^_^