I don't agree with Twistedmind on everything (for instance, I think the "43.75%" number is misleading; it's still only going to be 25% stronger than it previously was in Sanctuary, and it's still only going to be 15% stronger in Sanc than in Serenity - the 43.75% diference is between the old value in Serenity and the new value in Sanctuary, which doesn't make any sense to consider when you're talking spell selection).
However, the rest of the above post is 100% accurate, incredibly insightful, and should probably be required reading. Especially this:
If there is any encounter with some party damage, using spells like heal, flash heal, greater heal, binding heal, and renew would be inferior enough to prayer of healing that you are better just waiting for the ideal prayer of healing circumstance to arise and then just use that instead.
We've been there before (t11). It was powerful, but not very fun. This is why PoH needed to be nerfed. (Sadly, they decided to go after our mana first, leaving us crippled for a tier and a half...this is probably what will happen again, because they prefer to nerf via mana cost and buff via heal size).
And this line:
Despite our healing compared to other classes, we are strangely balanced. We may not compare to their hps, but our toolkit does have a working balance with itself.
Again, this was an issue I talked a lot about in t11 because our internal balance was broken.
Right now, for Holy, the internal balance is working pretty well, aside from the fact that you can't switch fluidly between Renew/Cascade/triage-and-HoT style and full-strength PoH/CoH. Buffing PoH across the board does nothing to change that lack of fluidity - it just breaks the balance so that you always prefer PoH.
For Disc, the internal balance is already skewed - but it's skewed in PoH's favour.
Buffing PoH just makes it worse.
(Not sure about the issues with Spirit Shell interaction. If PoH caps SS in one cast, there's something very wrong.)