The latest update on the mastery has this
* Restoration - Increases the potency of your heal over time spells by up to 20% based on the current health level of your target (lower health targets are healed for more). Healing increased further by mastery rating.
At first I didn't like this, but when I thought about it from the perspective of the planned healing changes in cata I think this is pretty nice to be honest. In an aura fight it's of course massive, especially with cata's focus being on filling and emptying health pools rather then people being alive or dead. It does mean that even with fast incoming damage, the rate of health decay slows as it approaches death, which gives people more time to react.
It's a very passive ability, which is sort of too bad, but it does make it easier to balance. It would be interesting to see a fight where at a certain HP% a druids hots would essentially stop incoming damage, and so due to mana constraints being introduced again in cata you might not want to keep the whole raid at 100%.
Seeing as I've gotten into the beta, I'll try to run some tests and give you guys actual numbers. It might be slightly challenging as I don't think mods are allowed yet.
Edited, Aug 2nd 2010 11:24pm by Xsarus