Tanking Mechanics, Revisited

Protection Paladin tanking, notably Holy Shield, is flawed in design.

Protection Paladins require the monster to be MELEEING them to generate sufficient threat.
To generate sufficient sustained threat, they must take enough damage and continue to be melee'd.
Feral Druids and Protection Warriors must TAKE DAMAGE to generate sufficient threat.
to generate sufficient sustained threat, they must continue to take damage.

This basic concept is fine until you factor that Paladins have no way to pull casters to them barring LoS, and cannot interrupt spells to force caster mobs to begin meleeing them. Sure their spells will require damage to be healed and thus feed them mana, but a very significant portion of their threat is from actually blocking attacks. Warriors can interrupt spells or reflect them, and intervene away from melee monsters to either a caster or a healer. Feral druids can also temporarily avoid melee damage while cutting out a caster by Feral Charging. All tanks have a long cooldown stun.

The problem is compounded with the fact that paladin tanking is absolutely shut down by many more things than any other tank.

Silences, Mana Burns, Fears, Knockdowns, and spellcasters all affect paladins to a greater degree than other tanks, by virtue of either base mitigation values, or ability use. Very few fight mechanisms specifically target rage(RoS), but many target mana under the idealogy that actually 10k~ mana healers/dps will be getting hit by it. An oversight, perhaps, but a very large one.

As a more minor note, Holy Shield should most definetly be taken off GCD. Paladin cooldowns overlap in an irritating manner ( 8 sec, 8-10sec, 10sec ) and as an extremely important tank mechanic, I don't think holy shield should be a holy spell, nor take up a GCD. This is not outside the realm of possibility, as Warriors already have their block ability off GCD.