"-Challenge--this must be very carefully balanced. It must be sufficiently tough to make the player want to keep trying, but not too hard so the player gets frustrated quickly. Having different skill levels is helpful, but you still have to balance the game so it is progressively harder. It should be fair."

I'd like to add my hate for "balanced play" that punishes success. I was just playing 18 Wheels of Steel Across America and I purely hate how your truck suddenly handles like crap and loses power simply because the numbers in your bank account are larger than somebody thought you should have.