For comparison, a game where characters go around and make friends would be possible, but would be hard to structure so that it contained the four elements. Successful variants are games where one builds chains of hotels, competes in the computer business, explores the universe to prevent a star from exploding, etc. Such games can be simple. In the 1970s, a computer game that allowed only decisions controlling taxes and buying grain was a success because it offered all four of the important game factors. Rules provide the skeleton or framework into which scenarios are fitted that provide for difficulty, challenge, reward and control such that a satisfying game experience occurs.
All rules have three parts.
The first is the style of the writer. One of any author's greatest strengths is his ability to write directly to the taste, understanding and needs of his audience. That writing style is common to all successful products in the role-playing genre, from novels to computer games, and has nothing to do with content. The second part of a rules book is content. Content has two aspects.