Tom,
Don't know that it was due to problems. I think the evolution in "03 was one of the largest in the past few years. In '04 it seems like they refined them a bit again with larger rotors (again) and other changes to the rear brakes.......
I misread your post and thought you meant changes from '03 to '04......
Like Jon said, since '02 they changed a pile of things, but most of the '02 bikes probably have the upgrades in mechanicals like trans & convertor done.
One thing to remember, if you go to an '03 or newer, they changed the wheels and hubs, so there are fewer spares hanging around to fit them.
Tim