I've borrowed this from Shattered World (see links). The author of that blog is using it as a basis for constructing a new DnD campaign setting - something I would love to do one day!

They reference Rich Burlew (from Giant in the Playground) and his 11 basic assumptions of (existing, main-stream) DnD campaign settings:

• Humans dominate the world.
• Gods are real and active.
• Magic is real and can be used by anyone who learns it.
• Opposite alignments fight each other.
  Arcane and divine magic are inherently separate.
• The wilderness is separate enough from the cities to justify 3 wilderness-oriented classes.
• There are hundreds of intelligent species of creatures, but 99% of them are considered "monsters".
• Arcane magic is impersonal and requires no "deal" with a supernatural being.
• Beings from other planes of existence try to influence the mortal world, usually on behalf of gods/alignments.
• Magic items are assumed to be available, and game balance proceeds from that assumption.
• Magic is consequence-free.

One of the reasons I'm putting this here is to keep hold of it for when I finally come to create my own campaign setting - it seems like a good trigger for important questions about the world. Another reason is that it highlights the kind of questions (and answers) one must ask about anything they are creating - from characters to cosmos. Thus far, the one thing I have learned in my years trying to write; reading about writing; and thinking about writing; is good writing comes from questions.

Questions begat few answers and more questions in a way the old testament would be jealous of. From the benign -"Why does Sammy always wear her hair up?" - to the ridiculous - "What if time travel was not only possible but available?". Both these questions allow us to know our creation even better and as such describe it better to someone else!