Okay, I'm sitting down to write one of those "oh, god, my GM handed me ten pages of homework to read before starting the campaign" things. (D&D 5e, med-low magic 4-race fantasy.) So far what I've got is
setting: rough geography, broad-strokes history (esp. recent), the gods, the peoples;
character options: playable races, playable classes, how alignment matters;
gm-matters: how I use TBIF-->inspiration, roles I need players to take;
some maps and art.