Maybe the "killing blow" bit is an overreaction, but the utter destruction of our mastery is far from it. The stats as they stand now are fairly balanced, and there are some convincing arguments to stack crit or haste (You should pretty much ALWAYS stack one stat, the question is just "which"). Post patch, my best estimate Mastery will be worth about .6 crit, so basically completely worthless.
I just think that a spec with a worthless mastery, relegated to scaling based on crit as a healer, and with it's signature abilities and utility being nerfed so heavily, is pretty much dead.
How exactly is Mastery worthless though? Compared to Crit, it benefits DA in a 2.5:1 ratio and SS in a 0.83:1 ratio.
There's a reason why almost every top Disc Priest stacks Mastery, and that's because Mastery is far too overvalued right now. 5.2 appears to be changing it so that the choice between Crit and Mastery isn't so clear cut, and Disc Priests are probably going to have to choose whether they want to have more reliable and consistent DAs versus having bigger SS numbers.
Even with the nerfs to SS, its output and burst potential will still provide far more utility than any other CD in the game, simply by the expedient of being able to use it pre-emptively. To keep the interaction between Mastery and SS the way it is now would simply mean we'd once again scale better than the other healers, something which isn't good for the balance of the game.
Compared to crit it benefits Prayer of Healing at a .83:1 ratio, and other heals at a .11:1 ratio. You don't use just the Divine Aegis portion of a heal. If you go by your logs, DA is 30% of your heals, so to raise your total healing 1% by raising divine aegis you need 800 rating. You need to look at it based on either the log weights or entire spells, not divine aegis alone
The fact is that, post nerf, the amount of Mastery required to raise your TOTAL healing by 1% based on any reasonable distribution of spellcasts and yes, overhealing, is going to be significantly higher than the amount of crit or haste needed to do the same thing. By significantly I mean anywhere between 30% and 1000% depending on the spell breakdown.