Thinking back to my Screenwriting and Theatre days, one of the basic principles of good fiction writing is also advice more RPG Setting creators need to take to heart: "show, don't tell". If it's not going to show up "on screen" in some way (in the case of an RPG that would be "at the table") then it either needs to be reworked, or removed from the story. Long passages about the geography and history of some Tolkien Clone fantasy world - this actively makes a game worse by diluting the usable material.
Players should also consider this when writing their character "back story" - eg. don't write your characters back story. Write your characters appearance and personality instead. This is the stuff that the other people at the table will actually see. Few people want to read 50, 10 even 2 pages of backstory for someone else's character. Most people don't even want to read that much campaign world description.
So how do you Show your RPG setting without reams of text for people to plod through? Make them encounter it as part of the gameplay right from the beginning.
Races and Classes
Right at the beginning of your game when players are making their characters you can start introducing your campaign world by choosing which races and classes the players can start with. The Dark Sun campaign setting is a good example of this - through picking a character you learn about this world of gladiators, savage elves, insect warriors and dangerous magic. The Forgotten Realms as expressed through D&D 4e does a terrible job with this. There are so many different races and classes you're left with no real sense for what the campaign world is like. There's too much choice and you can't see patterns that suggest a cohesive setting.
If you're using a game like 4e or Pathfinder or even original D&D with lots of Dragon Magazine articles with extra classes - it's worth thinking about restricting the starting lists of races and classes to help introduce the campaign world. As new areas are introduced and explored these new options can become available to the players.
Like Races and Classes the gear you let players look over and equip their characters from is a great opportunity to introduce more about your campaign setting. Consider an items list that would include:
- Mallet and Wooden Stakes
- Holy Symbol
- Silver Arrow
and another that included these instead:
- Billowy Robes (Bedouin Style)
- Sand Goggles
Putting them all together and you get no real sense for a campaign world. On their own you get a feeling for the setting without any background information or geography lessons.
Other Ways to Introduce Setting
Any other choices the players make in creating their characters are great opportunities to introduce more setting details. Spells, Minor Magical Items, even a list of suggested starting names can help.
Print out some artwork and glue it to your notebooks. Make your own DM screen with images you think set the right tone for your campaign world. Get some character sheets done in a style that's less vanilla accountancy and more evocative of the specific genre you're running your game in.
I'm also very much in favour of these DMing / Adventure writing techniques that were more en vogue in the earlier years of the hobby:
Buy the Old Timer a Drink
Each time you do, roll on the random table for some bit of information about the area the adventure will be taking place in. These might be reports of monsters that have been seen, local legends, news about what other adventurers are doing, or even some bit of info about the local nobility that could tie into the politics of the region. What's important is that this is something an NPC is talking to the Player Characters about -- not some dry bit of text you're asking the players to read, or you're narrating at them.
Maps and Keys
Maps as props that the players have and can refer to are great.
Maps that the DM uses with information about not only the monsters and treasure in a certain location, but also a sentence or two about the scenery are one of the best ways to incrementally show the players the campaign setting. Even a couple of words can go a long way.
"You go to the Inn" doesn't tell me a lot.
"You walk through the rain to the moldy Inn" says some interesting things without taking too much time.
Less is More
I understand that if you work for a big RPG publisher you're getting paid by the word, and it's in your best interests to pad out that text and make your setting info as verbose as you can. If you're a regular gamer though... that's not going to help you. Focus more on what you'll actually use at the game table and I you'll be able to enjoy the work you put into them even more.