Backgrounds for RPGs are something I think about a lot. As a player and a referee I believe it is a strong background that makes a good RPG. The question I keeping asking myself is what then makes a good background.
The first thing that struck me when I started thinking about this was interconnectivity. A game needs all of it's background parts to connect. If the players are going along a linear scenario they support material needs to fit what the players are doing. In fantasy games there shouldn't be a lot of background information in the form of something like a gazetteer. Information was passed by word of mouth. Bards and skalds pass on local news, they know what is going on and where things are. Most importantly in history, bard and skalds pass on the legends of the culture.
Myths, legends and lore about gods would be a strong influence on the population even if there is little to back them up. In a world where gods manifest themselves and the warriors of legend have only been dead for a generation, their impact would be even greater. Most people would know the stories of the gods or saints during the middle ages. Such stories would be recounted by fathers to sons and from priests to their flocks. Just having a list of gods and their areas of interest is good but games like D&D don't venture much further unless you buy another book. In reality, stories of gods show that they are much more complicated that most people consider them to be.
Maps play an important part in most games. It is something that the players and the GM use as a storytelling prop as well as gauge how far the players have to travel to get where they want to be. A number of GMs that I know spend a lot of effort on a map. Whilst this is laudable it would unlikely to have happened in reality. Maps were few and far between and typically highly inaccurate. If the players have anything at all it would probably be little more than a rapidly drawn sketch. In the game I am working on at the moment, I have a number of maps in mind. The game is set in the London Underground. The Tube map is iconic. Every station has multiple copies and many diaries have a copy of the map in them. The trouble is, in a post apocalyptic game, such maps are massively out of date. The Tube map is symbolic. The gaps between stations have nothing to do with actual distance. Accurate maps are few an far between and are not available to the public. Then there are post ruin maps. These are made by the survivors. This means that inaccuracies are going to be everywhere whilst perhaps being more realistic that the symbolic maps.
Players need a good reason for their characters to work together. They need something that ties them together as individuals in the real world. Whatever goes on in the game has to mirror, in part at least, what the players see as the view of the real world. I think this in one of the reasons why evil campaigns don't work so well. Whilst I have been in purely military campaigns there are plenty of players to whom this does not really appeal. Following a dogmatic chain of command's orders can be hard to take. Playing rebels is much more fun for players than playing government forces.