Stars Without Number is a sci fi rpg, which is free in its pdf format.
I would suggest picking up SWN, reading the worldbuilding chapter, and generating a sector. You will immediately notice how all of the details are based upon potential PC interactions -- the game is designed to generate cool interactions. These details, in turn, suggest other details to the creative mind, which then flesh out the setting.
Each world has two "world tags" that suggest, in turn, specific examples of friends, enemies, places, objects, and complications that arise due to the setting. Every setting should suggest these things at a very minimum, and SWN does an excellent job of showing how to generate them.
Now, ignore the sample sector. It's not as fleshed out as you want.
But think about how you can get across those PC-oriented details to the players without revealing the secrets behind them. And there, in a nutshell, is what you want -- consistent details that lead directly into the meat of the action. You can add as much extra detail as you want, but the core must always lead to the reader thinking how cool it will be to be/encounter/interact with the elements you are presenting.
If you think it would help, I could send you some sample setting material I am working on for a SWN game (pdf format). One part of this process, for me, was stealing evocative images from the InterWeb, and then using them to add detail. For a published setting, you cannot do this, but you could get some artwork developed, and then use it to spur further creativity.
Also, describe whatever should provoke strong emotion in the setting: What is desireable? Horrible? Frightning? Loved? Abhored? These things offer hooks the players (and GM) can hang their hats on!
Good luck!
RC