I started my DF game with:
- some players. Not all of them, just some. Five guys started, and now we're up to 7 with some talk of adding more if we can think of local folks we'd like to spend Sundays with.
- a list of playable races and stats for them. Those races had about 1-3 sentences of fluff text.
- a list of acceptable templates (for you class & level folks, read as "class" or "profession.")
- a dungeon to bash on for a few sessions.
- a map from the civilized spot to the dungeon, in case they went a bit astray.
I was able to put that stuff together in only a few weeks, an hour here or an hour there, as needed. I could have done all of it in a day if I needed to. Thanks to using GURPS Dungeon Fantasy, I was able to do this stuff by simply picking out what I needed and discarding what I didn't. I didn't need to make much up.
I didn't have:
- religions mapped out. I put "Good God" as a placeholder on the cleric's sheet and he ran with it, all, Good God Y'all style. It went from placeholder to real name.
- a name for the world. I still don't.
- a map of the play area. I still don't.
- a name for the megadungeon I was planning to add ASAP. Or anything more than a vague idea of where it would be. I wasn't even sure if the game would be "convert AD&D and Rolemaster stuff and roll around from dungeon to dungeon" or "have a base and a megadungeon" or not. Not 100%, although I was leaning to the latter.
- most of the details of my game world you might know from reading our session summaries.
I write up monsters as I think of them or need them. NPCs, too. I map just ahead of the players. I write down the crazy crap they say and make it true about the game world. I make crap up and make it stick. I rule stuff off the cuff ("rulings not rules" is a good way to roll during a session) and then I write them down and use them again if we like the results (because that's how you make rules).
The upside to this kind of approach is, you get the game rolling quickly. Your enthusiasm is up. So is that of the players. You're three sessions in and on a play schedule before you realize it. You left so much blank you can afford to let the players fill it in. Strike while the iron is hot, don't wait until you've figured it all out.
Why not? Takes too long. You have to write it up, then teach it to/pass it on to the players (instead of letting them decide what it is), and then make sure what's going in fits in with what's there already. Why not just start playing?