Holy pallies love their mastery shields, and 5.3 will see a slight reduction in Illuminated Healing Shields.
For many holy pallies, Illuminated Healing is their number one healing spell by a large margin – 30-40% of total overall healing, so even though a 2% nerf seems small, that will have a much larger impact overall.
Here is the nerf:
Mastery: Illuminated Healing now places an absorb shield for 10% of the amount healed, down from 12%.
Not much discussion on the change yet, however Pallies are overpowered (along with Priests) currently, so this is obviously to combat that. And non-shield healers will like the fact that one of the biggest absorbs in the game is being slightly nerfed.