Disclaimer: I run strictly run 10-player raids. The point I am making here is a point about how spell power coefficients distort how the game is actually played and the decisions we make. I believe the particular example doesn't necessarily work in 25-player raids.
Recently I was reading a blog post at Penance Priest about how to heal as a discipline priest. The central theme of the post is very good advice, and advice that actually applies very well to most classes these days, not just discipline priests.
But I'm not really going to comment on that post, I'm only bringing it up because it reminded me of a recent change I've made in the way I play. The post casually mentions that Renew is terrible. And it's natural that it mentions this casually, as a sort of passing joke, because every discipline priest knows that Renew is a bad spell for discipline. We know this because we started this expansion with around 2k spell power. Renew heals for:
(1400 + (1.88 * spell power)) * 1.05 [Twin Disciplines] * 1.04 [Focused Power]
That's 5635 healing with 2000 spell power. You could get it to 6480 if you took Improved Renew, but Renew is bad, so you don't take Improved Renew.
At the same time, Flash Heal heals for for:
(2040 + (0.806 * spell power)) * 1.04 [Focused Power] * (1 + crit chance * .95)
Flash Heal healed (and shielded with Divine Aegis) for 5241. So if you are willing to wait 15 seconds for your healing you could get about 7.5% more. This is not much of a trade-off. Even if you ignore crits and spend the talent points on Improved Renew you would need three Renew ticks to equal a Flash Heal.
But we don't have 2000 spell power anymore. In fact, we can have more upwards of 4500 with a demonology warlock in the raid. With 4500 spell power Renew heals for 10800 while Flash Heal heals for 8130. Now if we ignore crits and spend the talent points on renew we need 2.4 ticks to equal a Flash Heal. Renew improved about 25% relative to Flash Heal over the course of the expansion.
Even compared to Power Word: Shield, Renew is still looking like something you might want to cast sometimes (given that Weakened Soul is a thing). I've mentioned before that I realized at some point that I didn't want to be using shields to raid heal on Blood Queen because I want to save them for pacts. What do I do with my GCDs when I am not shielding? I was casting Flash Heal at first, but now I cast Renew. What do I do when one healer is ice tombed and the other one has unstable magic on Sindragosa? I cast shield on anyone at low health and pre-emptively throw renews around. What do I do when it's almost Bonestorm time on Marrowgar? I put Renews on people and keep shields for when I see who actually needs them.
In other words, I behave the way that I think the developers imagined I would behave when they were designing things. Renew is a preemptive heal against damage I know is coming, Penance is to bring someone's health up, Shield is for emergencies and to deal with attacks that do enough damage to actually threaten to kill people in a couple of seconds. The nature of the spells, ignoring the particulars, suggests this is how you would want to play.
It is the particulars that made Renew useless. Power Word: Shield is still a better spell, and Flash Heal still has lots of use. But with sufficient spell power, Renew has lots of use as well. Discipline priests rarely use Renew because we all took it off our bars so long ago. But as spell power has over doubled in the expansion, the value of Renew has radically changed compared to the alternatives. As I've said before, the coefficients of spells should bear a resemblance to the power - or intended power - of the spells, not to the duration or cast time. The result of the current system was the extreme distortion of the power of Renew and a "generation" of discipline priests who don't look twice at a meaningful part of their toolbox.