There's so many good points in here I don't know where to begin.

I agree with making bronze easier and gold more difficult. That would keep all the OCD people happy by having to pour hours into the game to get all golds. It would also keep the newbies happy. They can go through the game and still get all the cool unlocks without "wasting" 20+ hours replaying each track to get it down to a t. That way, everyone's happy.

I still don't agree with locking extra stuff to very difficult levels. Surely the reward for spending hours playing and getting really good is itself. You can smile that grin you get when you nail a fast lap or hit a really high zone.

Pulse does this very well.