Resto druids are currently dominating the healing meters in a large part due to getting a 15-20% boost to their healing meters just from using this cooldown well.
Have you considered changing it to a mix of mitigation and healing? For example, have it reduce all raid damage taken by 10% and cut its healing output in half. That would give resto druids some of the mitigation they have been asking for, while lowering their dominance of the healing meters.
Edit: Fixed grammar mistake.