Old style is slow and delivered. You go through maybe twenty beats of the drum to deliver movement on each side. You move your shoulders first, then bring the movement down to your foot. Then you repeat the movements with the other side of your body. In the new style grass dance, everything is fast.
Some guys look good but the top half of their bodies and shoulders are dead.
The main difference between old and new is simplicity and complexity. Old style is simple, slow controlled movement. The key to good grass dancing is to dance on the tip of your toes. Your toes and the ball of your foot should be a springboard for your movements. When you dance flat-footed, your shoulders can't move.