A rich world is something that, layered into a game, will also increase its appeal to an older demographic.
The richness of Tolkien's world couldn't possibly be captured in a film. Most people agree, however, that the Lord of the Rings films did as good a job as was possible. It helped make the movie much more than just a kids' film.
To learn how to construct a rich world, take a look at Chapter 2.18, "World Induction Techniques;" Chapter 2.5, "Group Interesting Techniques;" and Chapter 2.6, "Group Deepening Techniques."