I keep seeing people posting in here as if they only have 1 set of gear for pally tanking. More than any other tank, we need a wide selection of gear swaps to fully realize our tanking role in instances. As my guild's only pally tank for Hyjal and Black Temple, I shift gear sets slightly nearly every trash pull, and drastically EVER boss fight. Each shift is to maximize my effectiveness as 1/25th of the raid for the fight at hand, which is the foundation of every single player in that raid. To that end, as always, gear selection should be based on more than just 1 fight type. I probably carry way more than I should in gear, but I believe that no protection pally should limit themselves to 1 set, or even 2 sets. I carry gear swaps to handle trash avoidance, trash AoE, boss avoidance, boss off-healing, boss 'dps' (when I am not needed as a tank or a healer), plus whatever resist sets I need for that nights raiding.

Every protection pally should choose items based on which gear set that item will go with, and then use that set for the fight at hand.

The above post is has it right with this card, and the Vengance card is a permanent part of my AoE trashing tanking set, but is not present in my boss tanking set, my healing set, or my resist sets. It creates the most noticable difference in Hyjal, in waves with Necromancers or Banshees, in that I can hold aggro from Banshees or Necromancers for well over half the duration of the wave, even though they are at range and in the AoE pit where I am. I can work the camera, and see the fairly frequent popping of the trinket on these ranged attackers, enough that by the time melee or single target casters get to them, I still hold aggro fairly well until 30% or so. Yes, I can build threat faster on targets through other spells or attacks, but if my attention is being held by 50-60% of the trash wave that is directly on me, any threat I can build at all on the ranged to keep them off the healers is critical.

No, I do not use this trinket on boss fights, since those go to much more avoidance, reducing the frequency of procing, and it would only proc on a target I am already generating massive threat against, and isn't present in any fight where I am not expected to be the focus of damage.

Note: I have been noticing a few bugs with this, in that it procs off of ANYTHING, and sometimes the logic of how it reflects the damage is less than ideal. The most glaring example of this was in heroic Bot a few days ago. One of pulls just before the 3rd boss, I got that Curse of Doom like effect (not the name of the effect, can't recall it at the moment). We cleared the pull and were drinking when the debuff ended, dealt me shadow damage, and it apparently string matched to find out who dealt me damage. The trinket proc'ed on the left channeler in front of the boss, who we hadn't pulled yet. This pulled the group obviously, which was quite surprising to my party members who hadn't see the vengance proc damage floating above his head.

Is this sustained or a spike, because I can break out high numbers but over the course of the fight I usually settle into about 500-700 TPS.

I would say very important. Even if you're only going into AD range at 3-4k hp:

a) You can (and likely will) still take some heals before your next hit. A flash of light that puts you up to 5k hit points for instance before the next hit.
b) Not all bosses hit for upwards of the 4200-5700 damage to kill you when you fall to ~3-4k

(Najentus average is ~3900, EoS hits very weak, Akama adds (if they ever get re-enabled) hit weak, Infernals on Anatheron hit less than that. Doomguards on Azgalor deal less than that. Murlocs on Tidewalker hit weakly. Sharkiss + Pet each hit less than that. Hydross adds hit less than that. Lurker adds hit less than that. Leotheras hits for less than that. Vashj hits for less than that, as do her adds. Phoenixes on Al'ar. Trash in Hyjal waves. Etc.)

c) Most bosses that do hit that hard have a somewhat slow attack speed. This means unless your healers are sleeping on the job, you'll get at least a couple of small heals between hits. Also consider, at your gear level I place you in early t5 content. Most bosses already aren't hitting hard enough to blow through your AD. At 18750 HP, AD would kick in at ~6500 hit points. Assume you're taking a boss that hits for ~6500.

Without AD you would have to be at 35% health to survive the hit. With AD you could surive it as long as you have 4550 HP, meaning you could survive that hit anywhere from 24.2% upwards. Even some hits that could potentially leapfrog AD can turn into saves. For instance, assume a 7000 point hit. 4900-6500 would be survived. 6500-7000 would be death. 7000+ would be a survive. 26-35% and 37.3% upwards would be survivable.

You can't often take 2 back to back hits without heals on a boss that hits for upwards of 7000 damage anyways. If a boss is hitting that hard, you're going to be spammed. If you manage to actually get down to 3-4k (that would be approximately 2 hits without heals) and then take another hit without getting a heal, you've got serious problems that aren't related to your talent tree.

d) Many bosses have periodic and secondary abilities that bring you low that AD can help with. (Whirlwind on Leo, Inner Demon, Bat dung on Vashj, pouding on VR, flame patch on al'ar. Phoenix dot on Kael'thas. Infernal DoT on Anatheron. Doomfire DoT on Archimonde. Environmental DoTs on Supremus. EoA dot.) Many of these things are things you can avoid, (Like doomfire, flame patch or volcano on supremus) Most shouldn't kill you outright (Whirlwind, Flame Patch etc) But occasionally an unfortunate series of events puts you at low health when you get hit with one of these things. Or a volcano spawns directly under you, or something like that. AD will give you a bit of slack to turn what might have been a death into a near miss.

AD is by far one of the few talents in the prot tree I could not get by without.

Is there any hard and fast rule about how much block equals a certain amount of armor in relation to effective health? Like 200 armor equals 40 block value, or something like that. I'm a bit unsure which rings i should be using. I have the top tier Kara ring, the 2nd tier eternity ring, and the 7th ring of tirisfalen.

I am using the eternity and the 7th, but I'm wondering if the 391 armor on the great protector ring is worth more than the 24 block value and 24 block rating on the 7th. Thoughts? If not not on the ring choice, then the general question of armor vs. block value for effective health.