The nerfing of Divine Storm to physical damage is not as much of a PvE nerf as people seem to think. It's also not the change I would have made if I was trying to rebalance PvP without hurting PvE.
So, how is it not so bad?
Sunder armor. This is going to be up, or something equivalent, on all bosses. Reducing armor will make physical DS do damage closer to holy DS, with them being equal at zero armor. In other words, it's not a huge boss nerf. Trash won't get sundered as often, but it's trash. We didn't want to be trash tanks, why fight for trash DPS?
Who is to blame?
Well probably a dev who thought ret needed balancing. But people seem to like blaming classes, so we'll find one of them. Let's see, well mages cried a lot, so how about them? Nope. This change is only a tiny nerf against them since they have such low armor. How about warriors? Bingo. This takes them from 0% mitigation to 40-60%. Blizzard did say they they intend for warriors to go toe to toe with paladins and death knights. This change helps with that without having a huge effect against other classes against which we have always been weak.
What would I have done?
Leave DS as holy. Instead nerf the crit damage multipliers. Art of War gives 30%, Righteous Vengeance gives 25%. Drop those down by at least a third and make them affect all damage from the abilities they affect. This would reduce burst significantly without hurting average damage much.
Change the debuff on Divine Shield. Remove the melee slow. Replace it with a 10-15% damage reduction. The melee slow worked when we were auto-attack with SoC. Now we have controlled damage.
What is this nerf not affecting?
I've heard complaints that this removes the spell damage coefficient. There was no spell damage coefficient. Or maybe it was zero. They're effectively the same thing, though still different.
I've heard that this makes it possible to dodge, parry, and block it. Those were already possible. It's a lot like SoC, except without the spell damage coefficiant that SoC has. A better comparison would be SoB, so let's go with that.
If there was no sunder, how much damage is lost?
It would be a few percent. It hurts, but it's within the range of noise, so you might not even notice. For a rough estimate, I'll assume that it is getting cut in half against boss armor, becoming essentially an extra attack. I'll call attacks A. Over a minute there will be about 17 auto-attacks, 10 CS (which I'll round to 11A), and 6 DS (for an original of 12A, but now with a loss of 6A). Throw SoB on top for 35% of auto-attack, so about 6A.
17+10+6+6 = 39 The loss actually comes out to about 15%. Keep in mind this is a pretty worst-cast scenario, a boss with 50% mitigation, no sundering, and also no judgements, consecrate, or hammer of wrath. Adding all those in would drop the damage loss from 50% to only 15% or less, taking the overall loss to less than 5%.