Prayer of Healing is getting a little bit of rebalancing in Patch 4.0.6. It's effect is being reduced by 15%, but Divine Aegis is going to have an additional effect on it. From the looks of it they thought Prayer of Healing was too powerful for holy priests, maybe a little less so for discipline priests, but they wanted to make Crit rating having a bigger effect on it for discipline.
So they changed Divine Aegis so that you get twice the effect out of it when you get a critical on Prayer of Healing. As things stand now, Prayer of Healing may hit for around 7k and put up a 2.8k shield with around 14 mastery. On a critical it will heal for 10.5k and put up a 4.2k shield instead.
With the changes that same Prayer will only hit for around 6k and put up a 2.4k shield. On a critical, however, it will hit for 9k and put up a 7.2k shield. The non-critical heal is 15% lower, but the critical heal is 10% higher. This change makes your Prayer of Healing worse if you have less than 52% crit and better if you have more. We aren't really hoping to see 52% this expansion, so this change is definitely a reduction.
The amount of crit you need for this to be a good thing is sensitive to the amount of mastery you have, but at all levels of crit and mastery we will have throughout the expansion, this is going to be a slight reduction in Prayer of Healing power. Since the glyph does not account for the Divine Aegis shield, it is an even greater reduction if you use the glyph.
I already had the suspicion that Prayer of Healing was too powerful, so I'm not surprised by this. They are also providing significant boosts to non-Prayer of Healing heals from Discipline priests, most notably Power Word: Shield, but also Penance and to some extent all other heals through the Grace change. I'm not unhappy with the changes, aside from worrying that the Power Word: Shield buff is too big and will result in an extreme buff/nerf cycle.
What is really bothering me, though, is exactly how much extra shield you get when you crit with Prayer of Healing. Say you have enough mastery to get 40% of your heal value as a Divine Aegis. In that case, if you critical with a spell your critical does 1.5 times the normal amount and then gets an extra 40% in the form of a shield for 2.1 times the base heal. With a prayer of healing, your normal heal + shield is 1.4 times the base healing. The critical amount is 1.5 time the base heal with a shield equal to 80% of that for 2.7 times the base heal. That means it is 1.93 times the non-critical value. Prayer crits for 93% extra and other heals crit for 110% extra. The relationship varies with the amount of mastery you have, with Prayer falling further behind in the value of crit the more mastery you have.
I don't understand why they wouldn't just pick the value that actually made Prayer of Healing crits the same as the crits on other spells. The current implementation just seems so inelegant.