I actually go over this here:
I think that post covers why I think they need to be toned down a bit.
As for Disc… honestly, Disc priest is not really OP right now. It’s just that every fight has some mechanic that’s just perfect for Spirit Shell. Council, Sludgefist, Denathrius (not sure about Stone Legion Generals) all the hard fights are just so crazy good for Spirit Shell. So in this specific case, it’s not Disc being overpowered it’s just the fights favoring the spec so much.
Paladin healing and cooldowns are perfectly fine, but their damage during Ashen Hallow is too much. You can outburst most DPS specs with wings+hallow and it’s not too uncommon for a good holy paladin to do 3k+ DPS which is absolutely insane for a healer.
Shaman healing is very high, but they do no damage. However, their strength lies in the fact that they have so many cooldowns and useful spells. DR in spirit link, healing in HTT, AoE mana regen, combat ress in APT (APT is the best talent on like every fight except for Council) and prior to the recent nerfs, not a single guild killed SLG without 2 resto shamans or 2 holy paladins. They also use absolutely no mana, same as Paladins.
I think I’ve said several times that Disc/Hpal/Rsham brings too much and are op, however that’s a mistake on my part. I should have been more clear instead of lumping Disc in with the two other specs. It’s not op, but the fights are perfect for it. That’s not to say Disc would not be strong without perfectly-timed Spirit Shells, it’s just not something I’m eager to see nerfed.