Try to do some research about the class you want to complain about. Shield Barrier stacks with Vengeance. Vengeance having no cap means insanely high SB absorbs.
DK's have 2 AMS's if specced.
Pallies have DP.
I know you mean a main (magic or all) absorb that is frequently used, yes we have that and you don't.
But we don't have anything close to the potential self-healing of either DK's, pallies or bears.
Don't know about monks.
DK's do not have 2 AMS. AMZ is not nearly as strong when it comes to a personal CD. Both on medium CDs anyways.
DP is a 1 minute CD. That is not nearly as usefull as a bubble to negate magic damage. That is, unless it is a situation like madness where the heavy magic damage is spread out enough (P2 dot) where you can have it up for all dangerous moments. I would easily take SBa over DP. Also are you forgetting demo shout? We have SBa and 50% of DP on the same CD.
In the end I would take damage reduction over self healing any day.
Justa heads up because I dont think you know this, my main is a prot warrior so I am not crying for a nerf because they are not my class. I can do H Yor without dropping below 80% health 1 healing and 1 tanking. SBa is really overpowered. I will admit I hadnt looked at the formula for SB and I thought it scaled on stam and strength not AP and strength but the concept of the spell is still amazing even after they "fix" vengeance.
The AM system works like;
Warriors have Shield Barrier + Shield Block > Demo Shout
Druids have Frenzied Reg + Savage Defense > Barkskin
Pallies have Word of Glory + Shield of the Righteous > Ardent Defender
DK's have Death Strike + Blood Shield > Bone Shield
Now there have to be different characteristics to how the classes work, they homogenized the specs but you can't do that with classes or in this case tanks.
There are two things that need fixing in my opinion.
The passive reduction on Def Stance is indeed too high, but this is not noticable on live because tanks don't really take damage anymore. With the change to Mastery and it working primarly for Crit Block spiky damage was pretty frequent, so good for now but I think it will prove too strong at 90.
The second is that Vengeance needs a cap, not just because of SB and the absorbs but because on any challenging fight tanks are beating DPS on the meters. Some even reforge out of avoidance because of this.