I always felt that Forza Motorsport had a great difficulty system.
It functioned on a Postive Reinforement principle.
The player was never penalised for playing through the game with as many assists as possible selected, but for each assist that was turned *Off* they would get a boost in points for every race they won.
It therefore became up to the player to decide what level of reward they wanted, against the level of difficulty they felt they could handle.
For example many drivers may leave ABS, and Traction Control on while turning off Stability Management.
Others might leave all the driving assists on, while turning the AI Difficulty to HIGH.
For Forza, this didn't affect Achievements, it just meant that some people would take longer to reach the appropriate levels than others.
It was this wide variety of options that helped make the gameplay in Forza so appealing to so very many players. You didn't have to be a top notch driver to handle the cars and run through tracks, but if you were a top notch driver you could turn everything off and still be challenged.