Maturity's Dawn

It's when you look back on the fantasy and science fiction games you enjoyed as a youth and wonder, "Wait a minute. How many of those settings and stories were some guy's Dungeons & Dragons/GURPS/Traveller/Vampire: The Masquerade homebrew campaign?" The answer is 86.7%, by the way.

Elevation Derivation

In order to figure out whether magic works better at high or low altitudes, you'll have to figure out if magic comes from the earth's core or the moon. Obviously it's going to come from both, right? But then you have to decide whether your main character wields both or neither, and that's going to depend on countless factors such as your mood. Actually that's the only one.

Addendum

Regardless, a society constructed along such lines will eventually require a census to determine the size of each contingent, and oh boy, I bet the people responsible will get into some real fixes.

Pun Conundrum

Q. What do you call a society organized into tiers based on magical ability?
A. A cast system!

We all enjoyed that one, didn't we? But you can't put it into your setting, unless it's a Shadowrun-esque Earth, because there wouldn't be a caste system there, just a cast system, so if you tried to tell that joke everyone would supply the answer and not understand that it's humorous. It's hard sometimes.

Addendum

And don't feel forced to include, for instance, goats just because somebody mentions the year of the goat. That can be another tantalizing mystery.

Take the Easy Way Out

You want to include astrological matters in your fantasy story but aren't sure about coming up with your own constellations, processions, retrogradery, interpretations, and so on? Don't! Just use ours! Let readers debate whether that means it's post-apocalyptic. They love debating.

Unbelievable Settings

You might think you're clever, having cats be entirely absent from your world. The problem is that people instinctively reject utopias.

Magical Constants

If gemstones don't have magical properties in your setting, many people will still think they do. Just look at them. They must be possessed of wondrous properties.

Fantasy Technology Reminder

I know you've been worried about it, but I'm confident it doesn't take all that much to invent and manufacture steel guitars, so fill your setting with them without stint. Or, if I'm wrong, consult the previous entry.

Arguments for Giving Your Science Fiction Setting One Moon

If most of the galaxy was destroyed in an interstellar war, what do you expect? The remaining one probably has all the wizards on it.

Arguments for Giving Your Fantasy Setting One Moon

1) Nobody cares about the moon unless the story goes there at some point, and going to multiple moons is something accommodated by only a few stories.
2) Plus you gotta name 'em and junk.

On Vanity

If a kingdom minted coins that have a picture of a dragon on them, does that make dragons more or less likely to want them? Or no change? This is the principle question the writer must ask when deciding the nature of dragons in the setting.

Shaping Up

You've heard of round worlds and you've heard of flat worlds, but have you ever heard of ribbon-like worlds? Probably, followed by ". . . seemed like a neat idea but I couldn't think of anything that it would do for the story." That kind of thinking is why we haven't colonized the sun yet.

Addendum

That aside, the future will almost certainly have either trains or the apocalypse. Possibly both.

Science Fiction Logistics Considerations

When considering whether your setting's transportation has developed to such a level that there are no longer surface trains, remember that people can grab on to the sides and shoot bandits or else have tense fights on the top.

Setting Aesthetics Particulars

If you have masers, is your science fiction necessarily a retro future? Some kind of maserpunk? Do the computers have to be quite large? Or can you just throw those in anything?

Meteorological Reminder

You did give rainbows different colors in a futile attempt to make your setting stand out, right? I'm including you, science fictioners.

Important Fantasy Worldbuilding Considerations

If we go with an arcane/divine distinction, and please remember that there isn't much reason to do so unless we're writing Dungeons and Dragons fanfiction, and if also we decide our fantasy setting can go ahead and have a magical internet, a defensible decision, I bet the clerics would have it, or if they both do, theirs would be better. Wizards would keep sending each other faxes.

Q&A Corner

Q. How do I get across that my setting is full of magic and grand deeds?
A. Slivers of dead gods are a standard currency.
Q. How do I get across that my setting has dragons in it?
A. Here's the first line: "That was the day I saw a dragon."
Q. How do I get across that my orcs are noble and cool and stuff?
A. Here's the title: Neo-Fantasy Is All Out of Ideas
Q. What comes after neo-fantasy?
A. Rachet fantasy, then palin-fantasy, then intimate fantasy (intasy), then tidal fantasy.

The Sun Is Obsolete

Remember to have your fantasy world orbit the beating heart of a god, the final resting place of heroes, the mixing bowl where monsters are made, or something fun like that instead of dumb atom nonsense.