When it was first implemented back in 3.1.0 PTR, it took 30% of all damage happening to everyone in your raid for the duration of the skill. This caused the skill to be unusable unless you were using using Divine Shield as the incoming damage would often kill the person trying to protect the raid. Due to this issue when it went live, they added a little addendum that you could only redirect a total of 150% of your own health. It became a bit more manageable in this form and didn't always instantly kill the casting paladin. It still required some careful planning when not used with Divine Shield, because if you have 30,000 health and Mimiron nuked someone in your raid for 100,000 damage in one shot, you'd be dead faster than you knew what happened.
Because of all of these issues with the skill, it isn't widely used overall. Those that don't use it are often afraid of mis-timing the skill and dying. Those that do use it put it to very good use and almost always in combination with Divine Shield to prevent actually taking the damage. It is actually so useful in some cases that Blizzard feels they're having to balance around the skill in raids. Currently on the PTR, Blizzard is trying to adjust it for both types of people. They want it a bit less deadly as well as a bit less powerful than its current incarnation.
Divine Sacrifice (as of build 10712 of the 3.3 PTR) - The effect of Divine Sacrifice is now party-only and the maximum damage which can be transferred is now limited to 40% of the paladin's health multiplied by the number of party members. In addition, the bug which allowed Divine Sacrifice to sometimes persist despite reaching its maximum damage has been fixed. Divine Sacrifice will now cancel as soon as its maximum damage value is exceeded in all cases. Finally, damage which reduces the paladin's health below 20% now cancels the effect early.
This is from the PTR patch notes, so it's a little more wordy than it should be. Yes, Divine Sacrifice will still redirect 30% of the damage from everyone in your party, but only in your party. It also has that 40% multiplier on the total damage relating to your party. So, if you're sitting in a party all by your lonesome, despite being in a 25-man raid, this skill would do nothing by itself. If you're sitting in a party with one other person, then it starts doing something. Since there are two of you in that party, you'll be able to redirect up to 80% of your total health, but only affecting that one other person.
Also, you'll notice that if you're caught without a bubble up, the effect will end as soon as your health dips below 20%. Now, don't get too thrilled. It doesn't say it stops at 20%, but as soon as you've taken damage to reduce it below 20%. This means that if you're at 6,001 of 30,000 health and you get 7,000 in redirected damage then you'll still be dead. However, for smaller bits of incoming damage like the blizzards on Sapphiron you should be fine.
A lot of you will be looking at this and shouting "NERF!" and rightly so. That's because I haven't given you all of the information yet. They've also changed the talent for Divine Guardian
Divine Guardian (as of build 10712 of the 3.3 PTR) - This talent no longer increases the amount of damage transferred to the paladin from Divine Sacrifice. Instead it causes all raid and party members to take 10/20% reduced damage while Divine Sacrifice is active. In addition, the duration has been changed to 6 seconds, however the effect does not terminate when Divine Sacrifice is removed before its full duration.
Again, this is a patch note, so it only mentions the stuff they changed. What it does change is Divine Sacrifice's duration from 10 seconds down to 6 seconds. In exchange, you add a flat damage reduction of 10/20% (depending on talents) to everyone in the raid. It also lasts for the full 6 seconds regardless of Divine Sacrifice. This means that if you meet either of the conditions it takes to cause Divine Sacrifice to break early, the flat damage reduction will still be active for everyone in the raid for its normal duration.
This gives you a bit more of a choice. You can now be tossed into a low damage group where you can pop Divine Sacrifice without worrying about taking much damage, but still cause the 10/20% damage reduction for everyone. Alternatively, you can be tossed into the tank's group and be a greater damage reduction when using Divine Sacrifice with Divine Shield. Also, if you end up taking a bit more damage than you want, there is the possibility of it canceling before you die.
The Light and How to Swing It tries to help Paladins cope with the dark times coming in Cataclysm. See the upcoming Paladin changes the expansion will bring. With Wrath beginning to come to a close and soon the final showdown with the Lich King will be here. Will you be ready for the assault on Icecrown Citadel?