Yeah yeah yeah, I know, grease is fun. Blah blah blah I've heard it all, my guild invented grease skating. Anyway, this is how grease should be changed:
1. Remove friendly fire.
2. Implement grease as per stated in the PHB, outlined below:
Current: reflex save or fall, balance to stand, no check for movement.
Suggestion: Reflex save or fall. Balance check to stand. Another balance check to enable movement. If save is successful, balance check. Failure on the balance check by less than 5 prevents movement. Failure by more than 5 results in a fall. Successful balance checks allow for movement at half speed. Monsters that stand in the AoE of the spell are considered balancing, and thus flat-footed. They lose their dexterity bonus to AC, and rogues gain sneak attacks.
Oh, and grease needs to be able to be heightened and apply the casting stat to the DC if this suggestion is implemented.