Suppose you were playing a dps class with two spells: Spell A takes 2 seconds to cast and does 20 damage. Spell B takes 3 seconds to cast and does 25 damage. Sometimes, however, Spell A gives you a proc that makes your next spell B instant. This would make for a pretty obvious rotation.
If you got 1% haste, you would do 1% more dps. If you got 1% crit you would do 1% more dps - but since critical strike rating takes 40% more rating to get 1% they might give you a talent that makes your crits do more to even out crit and haste. What should your mastery be? If your mastery was that you do 1.4% more damage per point, then it would be balanced with crit. Suppose, though, they thought that was boring, so they made your mastery only affect Spell B.
Spell B composes 23.8% of your damage, so if a point of mastery is supposed to increase your damage by 1.4%, it would have to increase the damage of Spell B by 5.88%. Haste, critical strike rating and mastery are all balanced.
There is a problem with this situation, however. Suppose you get 10 mastery. Then Spell B does 58.8% more damage. That means it hits for nearly 40 a shot for a three second cast. Spell A still only does 20 for a two second cast. Your highest dps rotation is just to cast Spell B instead of casting Spell A and waiting for procs. Furthermore, mastery is now *way* more powerful than crit or haste. You should ditch everything you can to stack mastery and blast away with your single spell rotation. The higher the tier your gear gets to, the more stacking mastery and spamming Spell B will outstrip a normal build.
Of course this is pretty easy to understand from a dps perspective. It's no wonder that no dps class has a mastery that affects only one of their spells. They didn't seem to realize, however, that this can have the same unbalancing effect on healing.
Discipline priest mastery doesn't literally only affect one spell, but it comes pretty close. Here is the percentage increase given by 100 mastery rating on my spells: Power Word: Shield - 1.01%; Prayer of Healing - 0.3%; All others - between 0.1% and 0.14%. Mastery is more than 10 times as effective at increasing Power Word: Shield than it is at increasing Atonement healing from Smite and more than three times as effective on Power Word: Shield as it is on Prayer of Healing - the only other spell that consistently affects. Compare this to how 100 intellect affects the throughput of my spells: Smite - 1.19%; All others - pretty close to 0.67%. I've talked about Smite scaling before, so I'll ignore that for now.
If I want to make my spells more powerful, I should obviously gem Intellect. Since it is also a better mana stat than Spirit, gemming intellect is the clear choice. If, however, I want to make Power Word: Shield and only Power Word: Shield more powerful, I should gem Mastery instead. Could I use the same theory applied in the dps example above, do nothing but stack mastery, and cast only Power Word: Shield?
Lets say I manage to get 90% of my items to have mastery rating on them reforge in the other slots. That would be 1974 mastery rating. Also I put mastery gems in all sockets (we'll go with two in a staff, chest, head and legs and one in other socket-prone slots) for another 560 mastery. I can get 115 from enchants and two 321 mastery trinkets. This is a total of 3291 mastery.
Of course my other stats would suffer badly. Sacrificing Intellect and Spirit for Mastery would really hurt my mana. But using the numbers from the PTR for Power Word: Shield, it would boost that one spell to 7.72 healing per mana and 25.7k healing per second. Right now my highest efficiency spell aside from Penance is Smite at 6.27 and my highest throughput spell is a virtual tie between Renew and Flash Heal at 17.4k.
Talking about the viability of this plan is hard to do. Let's start by asking an easy question: could I put out similar total healing numbers by stacking mastery and only casting Power Word: Shield? Each of my shields would be around 35.7k healing and shielding. Dividing the healing I did on a few bosses by that, I get the number of shields that I would have to cast to contribute the same amount of healing to the fight that I did on our last win.
Here they are: Halfus - 45.5 over 5:55; Double Dragon (Valiona and Theralion) - 120 over 8:38 (Not the cleanest kill ever); Ascendant Council - 71 over 6:47; Magmaw - 84 over 5:26; Omnitron - 89 over 6:24; Maloriak - 81 over 6:01.
So the highest rate I'd need to cast shields to keep up with my normal healing is one every 3.9 seconds, and a typical rate is one every 4.4 seconds. Would I have the mana to keep that up for nearly 9 minutes? I would actually have enough mana to keep it up forever. Assuming I can get a Rapture proc every 14 seconds, I would be very slowly gaining mana while casting at this rate.
So the final question is whether that distribution of healing would be viable for winning fights. You'll notice I didn't include some fights in my list, such as Conclave and Chimaeron. Healing those fights with shield alone would certainly not be feasible unless that shield was very big indeed. But casting nothing but shields is not necessarily a bad healing distribution for typical fights. Magmaw will always eat through shields with damage would have to be healed otherwise. Shielding Arcanotron's target and/or shielding everyone during Magmatron is fine. Shielding everyone before Maloriak does his fire breath would be a huge help. Shielding everyone on Magmaw and and Double Dragon would work out. Shielding everyone would work great for the important part of Council.
I don't think this is actually the right way to play a Discipline priest - not for 10-player raiding anyway. It may be precisely the right way to play a Discipline priest for 25-player raiding, and it will almost certainly be the right way to play for 25-player raiding in the final tier of gear where the difference between stacking mastery and having more reasonable stats will be more pronounced. Of course by then the crazy scaling on Smite will make Atonement very overpowered as well. Maybe bringing two discipline priests to raids will be a good idea after all, but more likely we'll see some hideous nerfs around at tier 2 or 3. I just hope that they realize that the problem is that they gave Discipline priests a mastery that only affects on spell, and no amount of tinkering with the power of that spell will really correct things.