My biggest disappointment in the game is the inability to customize the skateboard. They could have done so much with different designs and speed, handling and ollie height power-ups.
I know bikes become more important later, but no matter what bikes or motorized vehicles you unlock, the skateboard is the only thing you always have with you. So many times I needed to get the hell out of wherever I was and couldn't get back to my bike or go-kart, and boy was I wishing my skateboard was faster.