It looks like Blizzard is going to give healers something to do rather than react to raid damage in Cataclysm. I am looking at the sneak peak at the released talent trees and I see some suspicious talents that convert damage into healing.
Penitence (3 ranks) - Increases the critical effect chance of your Smite and Penance spells by 5/10/15%.
Atonement (3 ranks) - When you deal damage with Smite, you instantly heal a nearby low-health friendly target within 40 yards equal to 15/30/45% of the damage dealt.
Evangelism (2 ranks available) - When you cast Smite you gain Evangelism, increasing the damage done by your Smite, Holy Fire, Holy Nova and Penance spells by 2/4% and reducing the mana cost of those spells by 3/6% for 15 seconds. Stacks up to 5 times.
Archangel (1 rank available) - Consumes your Evangelism effects, instantly restoring 3% of your total mana and increasing your healing done by 3% for each stack. Also allows you to channel Penance while moving. Lasts 18 seconds.
Divine Accuracy (2 ranks available) - Increases the chance to hit with your Smite, Holy Fire, Holy Nova and Penance spells by 10/20%.
These might be aimed at PvP, but it looks like Priests might become shockadin types with a smart heal attatched. We will see, but this looks interesting in terms of the direction Blizzard is taking with Priests: heal when things are frantic, DPS when things are boring.
Shaman don't get as big a toolbox as Priests.
Focused Insight (5 ranks available) - After casting any shock spell, your next heal's mana cost is reduced by 15/30/45/60/75% of the cost of the shock spell, and its healing effectiveness is increased by 5/10/15/20/25%.
Druids also don't get the same breadth of tools as Priests.
Fury of a Stormrage (3 points) - You have a 5/10/15% chance when you cast Nourish or Healing Touch to cause your next Wrath spell to be instant cast and cost no mana. Fury of Stormrage lasts for 8 seconds.
So, it looks to me like Blizzard is trying to make healing more dynamic in that you can heal and DPS at the same time either via boredom or skill. If two healers applicants look really evenly matched in their healing output, a little DPS could tip the scales in favor of the better player. The shaman shock mechanic looks a little weak sauce, but I'm sure it will get retuned, unless they adjust the coefficients on Shaman shock spells to make them hit a lot harder.
As an aside, it seems that all the released trees have a damage reduction talent in reach of all three specs. Is this a sign that raid damage will matter a lot more as healers fight to keep their blue bar at least partially filled?
13 hours ago