Part of me feels that additional incentives would just be a band aid fix at best. People switched to DPS for a reason, they just found the DPS classes to be more fun. Offering incentives is basically making the game into a sort of job. You are basically paying people for doing something they don't like doing and any negativity that incurs during this can spill over into their fun time. Incentives don't solve the underlying issue.

The only real solution is to make tanking and healing classes more fun somehow.