Here is my 2 cents:
IMO all previous models worked initially, but failed once gears scale up. To be specific, upon each expansion, there are cases where some classes/specs do too much burst, which can be hot fixed by turning the damage into DoT. However, as the expansion continues, balancing became harder and harder because there are classes/specs that you can't turn its damage into DoT, which has become the new burst, while the ones that had been converted into DoTs become non-competitive. Eventually burst will get out of control due to scaling and people who need to stack resil just to not get killed under 5 seconds. However, since resil is balanced based upon PvP gears, so it does not matter how they stack resil, they can still be popped by people with mix of PvE gears like PvE legendary weps and trinkets.
The other outcome is that you will have healers that are simply unkillable and dot classes who stack resil to its max. Having healers running around the court ain't no fun for both parties. In all previous models, balance is always between class/spec becoming too burst oriented while having others that just won't die.
The 30% flat def is not going to fix this problem as you will need to balance it so that people can kill others. However, as gear scale up, the same problems will arise. At the start of the expansion you will have hour long battle as healers can top off members easily, making battles last for hours. Eventually, you will end up having people being popped in under 5 seconds again.
Itemization is only one piece of the puzzle as abilities are the dominate factor. The key is throughput dmg and healing. DPS should not be able to burst down people and healer should not be able to top off people instantly either. PvP defense should have a ramp up time and both healing and damage done should have a throughput control. Each dps ability should apply a short, but stackable buff, lets call it sideeffects, to the receiver and healing ability should apply a stackable debuff to the receiver in PvP so unecessary burst can be eliminated. The ramp up time of Sideeffects can be reduce depending on PvP gears offensive stats, while defensive stat is to reduce the ramp up time of the defensive buff.
This way, dot class such as aff lock can't simply dot everyone as this will keep defensive buff of enemies up at all time, while melee no longer required to seat on prays continuously as that will also keep defensive buff up. On the other hand, dps will be able to unload huge amount of damage on players will doesn't have defensive buff ramp up yet. Normal CC, immobility and slow should automatically ramp up the def buff while stun does not(stunned target who got attacked will get def buff from the damage received).