Short Version: Nothing to see here, Folks. Just keep walking. Reckoning is that way, slightly more useful now, you should check it out.
Long Version:
Divine Shield now acts similar to the Warrior's
Shield Wall ability;
Divine Shield
3% of base mana, Instant cast, 5 min cooldown
Reduces all damage taken by 50% for 12 sec, but increases the time between your attacks by 50%. Once protected, the target cannot be made invulnerable by Divine Shield, Divine Protection or Hand of Protection again for 3 min.
Because it does not absorb all of the damage, it does
not cause a loss of aggro and can safely be used when tanking.
Divine Guardian, thus, allows you to take 30% of the damage caused to your allies whenever you use this "Pally Shield Wall". In effect, you're providing a 'baby bubble' to everyone within 30 yards, taking the damage they normally would.
There are several gross oversights with this ability, and I am hoping Blizzard realizes what a useless piece of garbage it is. First, this is completely useless for tanks; no one in 30 yards of you should be getting damaged at any time since we AOE tank like nobody else.
What's that? Why not pop it just before a massive Boss AOE and save everyone? Sure, go ahead, for 2 talent points you can Divine Intervention without using a reagent. There's no way in hell we would ever want to subject ourselves to 30% of every hit delivered to every character within 30 yards. So there's actually no good PVE situation to use this in.
Tanking?
Useless
Boss AOE?
Suicidal
So, by that logic...
Prot?
Useless.
But what if you're Holy/Ret? Surely you can use this to baby-bubble to help the tank and off-tank? Besides the fact that you'll be taking substantial damage you are not geared to handle, your tank and off-tank should be geared well enough not to need a bubble. If you're Holy, you'd be better off spending the time and talent points making your healing better. Your tank will thank you. If you're Ret, you should be spending the time and talent points burning down the enemy instead of trying to protect people. Your tank will thank you.
Holy?
Useless.
Ret?
Useless.
So, for those of you at home keeping score, that's three strikes, one for each talent tree. Complete waste of 2 talent points, and it actually takes an ability we would want to use often (Shield Wall) and makes it suicidal. Good way to get yourself killed in PvP or Arenas too.
The only way this would ever become useful is if the damage was simply mitigated / ignored (IE:
"While Divine Shield is active x% of all damage taken by party or raid members within 30 yards is ignored.", as it is in the talent tree in my signature). No paladin should ever need to redirect damage to himself, and any situation where he would want to would result in so much damage he'd explode (or come very close, driving your Healers insane and OOM, whichever comes first.)