The important bit of your setting is what actually sees the light of day, and gets used and interacted with. The players determine what's important, because that's what they actually see. You can have all kinds of other stuff, going on in the background, but most players aren't going to interact with it. IMO, it's usually unnecessary detail, and distracts from what's going on if you try and draw attention to it in a non-organic way. I mean, you might have this whole political scene you've established, but most players are just going to ask about which of the guys in funny hats is hiring them.
The important details, the ones I write down about my setting, are the ones that come up in play. Sure, you have to sketch out general ideas - These people live here, they act like this, this is your plot - but the players will ask the questions they care about the answers to. Personally, I prefer to make up these details on the spot, write them down, and make sure the answer is the same the next time someone asks the same question. Then again, I'm more comfortable making up details on the spot and remembering them later than I am trying to remember what I've already written. The temptation to try and get across stuff they don't need to know (and usually don't care about) is too strong, if I've already written it.
For example, for my current homebrew... I've got a map of the world, a map of the region the PCs start in, and a list of important people's names and businesses in the town they start in. Racial write-ups (in broad strokes), and some information on the nations around them... and that's it. Oh, and religions, about four or five paragraphs each. Maybe ten or fifteen pages, all told, and I think I've got a good enough handle on the world that I wouldn't be too worried about wherever they went in the setting.