For the setting itself:

• Basically Fantasy - more low fantasy and sword and sorcery in tone - with other fun bits bolted on. "Warhammer Fantasy, as GMd by John Cleese."
• No specific rules of magic at a macro level, with many insular rules of magic at the micro level.
• Lots of different races.
• Anything that might qualify as science-fiction or the like should be of a steampunk or Jules Verne bent; this would include any theories about how the world exists in the solar system, the universe, and everything.
• Other dimensions for weird crap to come from or leak out of.
• A long and storied history.
• Puns.
• At least slightly humorous, in the style of Pratchett/Discworld, keeping in mind that most of the humor of the books comes from wry, pun-loving voice of the NARRATOR and snarky comments by the main characters... not because the entire population is half-knowingly running a Monty Python sketch.