3.1's incarnation is too powerful and has begun to be a major balancing factor for encounters, The D-Guard effect may last for the full duration of D-Sac rather than ending if D-Sac breaks early, And this may not be the final interation of D-Sac and D-Guard development as the PTR is ongoing
There's nothing surprising in there, the talent is intended to work as you would expect except for the break conditions of Divine Guardian. Of most importance from a testing POV however is that we finally have a firm idea of how the talent is supposed to work.
I'll try to explain this all.
The old (3.2) model of "raid wall" where you get to essentially Shield Wall your entire raid by bubbling to avoid the excessive damage was overpowered and we were increasingly starting to balance around it.
The new (PTR) model is that Diving Sacrifice applies to the party, not raid. It can absorb 40% of your health x your party size. If you are in a party of 5, it will redirect damage to you that is taken by the party up to a max of 200% of your health. If you are in a party of 2, it will redirect damage to you that is taken by the party up to a max of 80% of your health. If you take enough damage to drop you below 20% of your health, the spell is cancelled.
Divine Guardian reduces damage taken by the raid by 20% for up to 15 sec when you cast Divine Sacrifice. The damage reduction isn't directly related to the redirected damage any longer, except that it stops when Divine Sacrifice stops because you have absorbed too much damage or dropped your own health too low. (I understand the current PTR may not have DG stopping when DS stops.) The talent still affects Sacred Shield as a secondary effect.
Whether you bubble or not will not change the amount absorbed. If you do not bubble, then both DS and DG will cancel as soon as it drops you below 20%. We added this cap because we thought the self-gib feature of using DS / DG without bubbling was weird. However, the spell can still kill you if the damage is quick and massive enough. If you have DS up when a party member gets hit by a Mimiron rocket, you'll die before the 20% cancellation kicks in.
Example: Kallee the paladin has 50,000 health. She has the DG talent. When she uses DS in a full raid, she can absorb 20% of incoming damage to the raid until one of three things happen: 1) 15 sec expire, 2) she absorbs 100,000 damage total (from damage to her party only), 3) she drops below 10,000 health.
Yes, this means that in some situations you could get the full 15 sec duration if some part of the raid is taking damage but the paladin's party is not. The more typical case is a situation in which the whole raid is taking damage.
A related significant change is that all sources of damage should now count towards Divine Sacrifice. Before there were some situations, Twin Val'kyr for instance, where the paladin wouldn't take damage or wouldn't take enough damage from the redirection.
P. S. We have been trying to move more abilities from the party to the raid, but some just don't scale nicely and are either overpowered in a 5-player group or underpowered in a 25-player group. We may implement some kind of scaling mechanism for Cataclysm or just accept that a few abilities need to remain party only.
P. P. S. This has changed a couple of times on the PTR already and it's possible this is not the final implementation, so confusion is totally understandable.
So, the discussion moves on from 'How should this work?' to 'Should this be used as a Raid-wide Damage Reduction cooldown or as Damage Reduction for the Raid's Tank?'. Essentially, 20% Raid-wide DR or ~50% DR for the tank. Personally, the only further change I'd like to see is to move Divine Guardian up a tier alongside Divine Sacrifice, to make it viable as Ret raid utility.
And that's my last post on Divine Sacrifice for a while. Promise.