That 20% decrease in personal healing ability is more than made up by the increase in raid healing from Grace. A 9% increase of MT healing and 9% decreased damage is huge.

Of course, strictly speaking, it's a nerf for 5mans, where you only have 1 healer. But 5mans aren't that difficult anyway.

Few things for thought, I was curious as to your "base stats" used in your comparison Ellyh.

If I have understood Blizzard's intentions with gear in WotLK, most if not all gear will be identical. (when considering caster to caster etc) So, I don't understand your desire to give the Holy priest 100 more spirit? Unless your implying that they would gain that from gemming or enchanting choices alone. If your doing that though, you might say the Discipline priest would gem and enchant for more +spellpower. (perhaps narrow throughput of healing power provided by Holy talents)

Giving the Discipline priest more crit, does make more sense however due to talents. Most Holy priests only have 2 points into Holy Specialization vs 5/5 for this new Discipline spec. Thus you might increase the crit% difference further. Not certain, but I imagine that 20% more Intellect over a Holy Priest at 80 could be quite significant when factoring crit%.

Lastly, "Mark of Divinity" will be a tool that all priests have true. The way each uses it however could be quite different. Perhaps much of the lack in power a Discipline priest encounters will be made up by casting this on their primary target. Thus making their heals on the MT 30% stronger. It's been stated already that Holy priests will lean more towards raid healing, thus they would be much less inclined to benefit from this on a regular basis.

edit: spelling

A while ago I read a very interesting post about Arcane/Fire mages and mana regeneration. The main point was that the 5 second count-down to get back OO5SR starts when a channeled spell is started, not when it finishes. In that post the idea was to cast Arcane Missiles followed by Fireball to get a few OO5SR ticks in as the full cycle is 8 seconds.

The same can be done with Penance and Greater Heal. If mana ticks every 2 seconds then a cycle would look like this:
0s cast Pennance, IS5SR
1s Pennance tick 1
2s Pennance tick 2, 1 tick IS5SR regen
3s Pennance tick 3
4s 1 tick IS5SR, cast Greater Heal
5s still casting Greater Heal
6s 1 tick OO5SR, Greater Heal lands, cast Greater Heal number 2, IS5SR
7s still casting Greater Heal number 2
8s 1 tick IS5SR, Greater Heal number 2 lands, cast Greater Heal number 3
9s still casting Greater Heal number 3
10s 1 tick IS5SR, Greater Heal number 3 lands, cast Pennance

That means over those 10 seconds a Priest gets 4 ticks IS5SR and 1 tick OO5SR. Assuming meditation that means a Priest using that cycle has a permanent (4 * 0.3 + 1 * 1) / 5 = 44% mana regernation, a 33% increase in regenerated mana.

Edit:
Oups, Pennance has a 10 second cooldown. Calculations adjusted for that.

Well, as I said I pulled numbers basically out of thin air. Also I am assuming that they are not in fact using identical pieces or gemming enchanting in exactly the same way. Is 5% crit = 100 spirit budget wise? Honestly I don't know, I just pulled some "Plausable sounding numbers" out of thin air to start giving a sense of how the talents multiply out. In reality I think that the spirit totals are low as I can hit 770 spirit in my current gear which is a mix of badge and BT/Hyjal items. so I would honestly expect a dedicated spirit holy priest to be well over 1000 fully raid buffed. This will simply exacerbate the power differential in favour of the holy priest as I think it is a justifiable assumption that disc will be more crit focussed than holy and given the existing power differential between the trees and the healing mana regen talent that they will ditch regen (spirit) for crit rather than sacrifice spellpower.

I must admit that I had assumed that MoD would not allow it to function as a straight 30% intensity buff on a single target but would require the heals to be targeted on a different player for the talent to function. Even aside this I don't really buy the argument that it is more powerful for disc as the holy player can still abuse it in the same way to maintain his edge in raw power.

A few factors to consider:

1. They're changing healing/damage into spellpower. As part of this change, they're almost certainly changing the coefficients on healing spells to around 180% to 200% of their current total. They're also probably changing the coefficients added by 'empowerment' talents (they've already done so with the Druid Healing Touch empowerment talent) to be balanced against these new coefficients. This doesn't really change your basic argument, but it's probably worth noting.

2. We actually know the current alpha values for the spells, so don't need to guess. Greater Heal (Rank 9) costs 1290 mana and heals for 3950 to 4590 healing.

3. In actual play, our Holy Priest will almost certainly average a higher level of critical due to Test of Faith. Other than this, Holy and Discipline Priests have equal access to spell critical from talents.

It depends on what you call 'wasted'. In the sense you mean, Power Word: Shield is an extremely mana efficient way to heal a single target since it routinely has 0% overheal (even on dps). However, we all should recognize that the 0% overheal from Power Word: Shield is really at the expense of some other heal - PW:S isn't more efficient than Greater Heal. It just gets there first.

The same is true of the shield you're describing - but even worse. Think of the worst possible time to lay on an additional shield for the tank. I'd say it's right after a massive critical heal in a timespan where the tank has 25% more armor than he usually does.

They are changing Circle of Healing to be a raid-wide heal, presumably healing the 5 lowest health members within the radius. I have seen no indication they are changing Prayer of Healing - by all indications, it will remain a party heal.

In all of the cases you mention, losing one iteration of the damage reduction/healing buff for 1.5s doesn't come close to compensating for the significantly smaller amount of healing you're delivering via Penanace rather than Flash Heal.

Another way to understand Penance is to look at the difference between healing and damage it delivers. Its healing is 13% more than its damage. Normally healing effects are about 80% more than damage effects - meaning that either the damage on Penance is overpowered or the healing underpowered (or, as appears to be the case, both).

Approximately, yes. 10 Spirit = 10 Spell Critical. ~21 Spell Critical = 1%.

This is a reasonable argument - if you associate raid healing with Holy Priests and single target healing with Discipline Priests. Which simply isn't the case.

Holy Priests already outperform Discipline Priests by a significant amount in single target healing. And their WotLK talents are almost exclusively oriented towards single target healing.

If Grace is absolutely critical to your raid's success, then you're also faced with a situation where having the Discipline Priest heal the tank is a horrible idea. You simply lose too much from Holy for single target healing. Rather, you'd assign your Holy Priest to heal the tank, and your Discipline Priest would merely refresh the buff from time-to-time while being engaged with other, less critical, chores.

You also have to consider that "half a healer" is not the same as "half a dps". DPS accumulates seamlessly into a collective value - total damage done. Healing does not. Healing has to be coordinated, and the loss of efficiency resulting from this coordination generally means that bringing 'gimp' healers isn't all that helpful - it just ends up creating more overheal.

Moreover, because the actual critical heal is somewhat likely to have overhealed in the first place, what you're really doing is increasing the effective crit bonus from [insert small number here] to [small number] + 45%. I have a difficult time using that as an argument to stack crit, as people seem to be assuming. If anything, a discipline priest will have MORE need for spirit, because they won't have 16% clearcast chance and Serendipity to give them mana back.

That's at 70. At 80, 100 spirit is actually cheaper than 5% spell crit, as presumably the rating conversions are going to go up again, but 1 stat point is 1 stat point.

In a 25 man raid, Grace alone reduces the healing burden required by 17%, if you do nothing other than keep it up on tanks. (Well, assuming all the damage is on a small set of tanks, as opposed to the raid, so in practice that number is lower.) This alone would suggest that a discipline priest is worthwhile in nearly all 25 man raids (with, say, 7 healers or more), as his throughput + Grace's effect will nearly always work out to the same amount of healing done as a "pure" healer. That's if you do pretty much nothing else (which would be a good fallback if you have mana problems). I suppose you could spend the rest of your time raid healing (for mana return/occasional Grace stacks on DPS), but it seems like you'd certainly want the holy priests to be using CoH on cooldown before the discipline priests use their somewhat more limited tools to raid heal.

People saying that divine aegis will be fairly weak because it is applied immediately after a crit heal (so the tank is assumed to be topped off) and during a time when the tank has 15% more armour have either not healed Brutallus or have forgotten how hard he hits. Unless the model changes, bosses that seriously threaten tanks can and will drop them in 2 seconds. The tank is assuredly not safe when topped off, and cutting some amount off the next hit s/he takes is assuredly not a trivial benefit. (The armor buff should be up almost all the time anyways.)

We already have an example of a class that serves as "half a healer" - shadow priests. In many encounters (or the ZA timed run) a shadow priest can provide enough of an AoE healing buff to allow the raid to run with one fewer healers.

Disc priests are intended to be weaker healers than holy priests (or any other healer) - given the enormous power of grace, they have to be.

Regards penance versus flash heal, unless someone knows the coefficients of the two spells we lack the information necessary to come to any conclusion.

But not every fight is Brutallus, or even close. Brutallus is part of a class of fights where you really can't hit a tank at the intended gear level much harder without making the fight impossible (or requiring things like heal rotations). Patchwerk would be another example of this, or enraged Illidan (well, he hits hard anyway, but there's not much else going on, and there are long regen breaks during phases 3-5), or doing Twin Emps with a tank in full Wrath. Even Malchezaar couldn't drop a tank of the appropriate gear level (read: March 2005) in 2 seconds (maybe if he triple-crushed?).

If Divine Aegis is only useful for the Patchwerk/Brutallus of WotLK, then it's going to be 3 unused points for most priests for most of the expansion. Compare Inspiration, which even if it only becomes critical later is something people will take earlier in the expansion cycle.

My argument against it is that people are using Divine Aegis as a justification for stacking crit to 20-25% on hypothetical WotLK priest scenarios, using the argument that we'll all be using mage/destruction warlock gear anyway (my personal feeling is that we won't, and that the itemization unification is an excuse to make healer items that are 80% optimal for casters instead of 50% optimal, and vice versa). If Divine Aegis is the only reason you're stacking crit (and some small gains in Inspiration uptime, although if we believe the "discipline priests will be raid healing" argument then you're not getting much personal Inspiration uptime anyway), then it makes sense to compare the crit that it would take to get that return from Divine Aegis to other uses of the same item points: say, haste. And if you don't have the crit to use Divine Aegis, then there's really no point in it.

Also, I use a lot of parentheses.

I obviously can't say for certain, but I see nothing in the description preventing using MoD on any target of your choice. I'm not quite sure how Blizzard would prevent you from using it on your primary target anyway. (How would they know who you were going to be assigned to heal?) Also, as I said, Holy priest certainly can use it in the same manner, but wouldn't be as likely to do so. I mean If the Discipline priest is there why not make best use of their ability vs the Holy priest's?

Some have implied that a Discipline priest would gem/enchant for crit%. I would disagree because I don't think they would need to. As I stated in my previous post, a Disc spec will have 20% more Intellect than their peers even before gem/enchants. On top of that, I believe a greater propensity to have 5/5 Holy Specialization vs the typical 2/5. I wouldn't hold my breath, but there have been rumors of Force of Will being changed to effect heals as well. All added up, a higher crit% will happen by default with no extra effort.

As to a Disc priest's regen, aside from what level of mana return they end up generating from Rapture, the higher amount of Intellect will make what spirit they do have work better.

Another thought on Rapture to think on. The talent states that it returns up to 2.5% of healing done in the form of mana. What if for this purpose it doesn't matter if the heal was all "effective" vs "overheal"? On that same note, what if Divine Aegis's shield proc value is added into the equation? Rapture doesn't state DA by name, but all the spells that do effect Rapture proc DA.

My point with Brut is that divine aegis will almost always be useful - either the boss hits hard enough that the tank is not safe even at full health, or the boss hits weakly enough that you can use the shield to spend a little more time oo5sr before letting your next heal land.

I agree with Robble though in that I don't see crit-stacking to be a good way to go. I suspect there'll be enough of it just through int, talents, and the odd bit that shows up on otherwise regen-oriented gear. Haste will be better for throughput and spellpower will be better for mana efficiency (and spirit will be necessary for sustainability). I think divine aegis is worth the talent points the same way inspiration is - you take it and it provides a nice benefit, but you don't gear around it.

Holy DPS needs 200% crits and some kind of utility to be viable. Until it gets at least the latter it's never going to be more than a levelling/grinding build/5-person instance build.

I would guess that a disc priest will, like today's retadins, be a fixture in 25-person raids and almost nonexistent in min-maxed 10-person raids. Grace is one of those "the more healers pointed at this target, the more powerful it gets" abilities and IDS is similarly more useful the more people you have who benefit. Plus there is much less room for a "utility" healer with gimped output when you only have 2-3 healers instead of 7-8.