Here are the changes relevant for priests:
Regeneration while not casting (outside of the “five second rule”) will be decreased. We think that (1) the ability to cast heal over time spells and then sit back and (2) benefitting from a clearcasting proc that also gets you out of the five second rule both provide too much mana regeneration, even over short time periods.
Nothing much to say about this one. This is the overall nerf. We can just wait and see if the nerf-bat makes white damage or crit.
To make this change, we are reducing mana regeneration granted by Spirit across the board. However we are also boosting the effects of talents such as Meditation that increase regeneration while casting. The net result should be that your regeneration while casting will stay about the same, but your not-casting regeneration will be reduced. This change will have little impact on dps casters, since they are basically always casting.
Could this mean that a Discipline spec will be an even more viable way to go? The general idea with Discipline is to let go of Spirit in favor for Mp5. The way to heal as a Disc priest is also to cast more or less continuously anyhow. You want to keep Grace up all the time and your heals are also less powerful than a Holy priest.
The specific talents and abilities being boosted are: Arcane Meditation, Improved Spirit Tap, Intensity, Mage Armor, Meditation, Pyromaniac and Spirit Tap. Yes this makes these “mandatory” talents even more mandatory, if such a thing is possible.
I will be interesting to see the outcome of nerf-bat vs Meditation boost.
We are also taking a close look at clearcasting procs themselves. One likely outcome is to change them to an Innervate-like surge of mana so that the net benefit is the same, but healers won’t shift to out-of-casting regeneration so often
Spirit based (I assume) so a good thing for Holy spec (but probably a nerf overall since they change it)
We balance around the assumption that even 10-player groups have someone offering Replenishment. To make this even easier on players we are likely to offer this ability to additional classes, as well as make sure that existing sources of Replenishment are more equitable.
Making sure there is Replenishment in the raid helps a Discipline spec since Replenishment is based on intellect.
I have never played with a Discipline spec., but overall these changes could benefit (or be less of a nerf to) a Discipline priest compared to a Holy priest.
Official blue post