Hi there. I'm a druid main (impersonating as a priest), but I do have a Disc/Holy Priest that I frequently raid with in ICC. I've always felt, even with similar gear, that my druid was a lot stronger as a healer. After playing tree, it was almost painful to go to my priest because every time we'd wipe on content because of heals, I knew that I'd be able to make up the difference on my druid where I couldn't on my clothie healer.
I started out as holy, but I can came to abhor the spec because I just never had the direct healing to keep up a tank who was taking lots and lots of dmg. Disc was much better at that job (obviously), and I really started to fall in love with instant-cast PW:S. It's almost like giving every raid member tank health pools (except on AOE fights, where it gets eaten away quickly). On AOE fights, it's guaranteed to be eaten up and it's amazingly crazy HPS (through preventing dmg). It's also the fastest, biggest counter to death we have on a short CD. Someone at 2k health can live because the next huge chunk of dmg gets absorbed rather than killing them. Nothing else seems to compare to it (especially on fights like Lady Deathwhisper, where dmg is bursty and unpredictable).
So, all of this build up leads to a request for critiqueing of a
Disc/Holy hybrid spec. I'll still run a cookie-cutter Disc spec for dedicated tank healing, but I'm thinking of swapping to this spec for the majority of ICC encounters that I currently run (Marrowgar->Suarfang + Fester and Rotface).
I really wanted to keep CoH in this new spec, because there are fights where I really do appreciate it (Lady Deathwhisper, Stinky & Precious, Festergut) and I wanted the spec to be significantly different so I'd have a reason to swap to it. I lose the choice of Pain Supression and Guardian Spirit, which is a big liability for spots where tanks get hit hard (but I should be in Disc if I'm keeping them alive anyway).
In return, I get a combination of neat things. B&S, GCD shields for all the fights where people need to move faster (Bone Storm, Lady's D&Ds, Spores on Festergut, kiting slimes on Rotface); whenever I'm not the main tank healer (think paladin with beacon on the 2nd tank; love paladins for their niche), I spend most of my time pre-shielding the entire raid. I notice that PW:S used liberally dramatically reduces losing people, and in ICC, most fights can't stand the loss of anyone; DPS or otherwise. I don't get the mana return from deep Disc on the shields, so I guess this could lead to mana problems, but it seems like I already have them in Holy anyway.
Also, I could never stand to use Renew in difficult raiding because I always felt that not topping someone else left them very susceptible to bursty dmg death, but I could see dropping a PW:S on them followed by a glyphed Renew. Once you start getting into a Shield, Renew, Shield, Shield, CoH, Renew, sort of routine, suddenly you become extremely mobile. Whenever your heals aren't needed, you just go into a pre-shielding routine (if raid dmg is expected). The result is you could heal without ever stopping if needed.
Again, my main concern with holy has always been it's lack of strong tank heals, and I would never use this spec as a main tank healer, but I think it has some merit with 3-stack-serendipity-greater-heals. While I couldn't any of the FH/GH supporting talents, you have a bubble-heal every 15s that absorbs a respectable chunk of dmg.
One choice I deliberately made was to drop Improved Inner Fire for Silent Resolve; just made sense for me due to personal playstyle preferences. Anyway, if anyone sees anything I missed, or has a reason why this spec simply fails, please feel free to enlighten me. I'll also post my impressions of how it plays next week when I get back into ICC (we finished my priest's ICC run tonight).