Will, I really like your points; I was going to right very much the same! Your points on seeking the roads and the tactical decision are spot on.

I think the best way to look at it is not to create a disadvantage for driving on dirt and mud, but to create an advantage for driving on the roads. Don't slow the player down for driving on the mud, speed them up for using the road. Changing accelleration is a good idea too, especially for mud situations.