I wrote this awhile back to familiarize my old guild with paladin tanks, since it's something of a foreign concept in 'traditional' raids.
A Synopsis of Paladin Tanking
I figured since protection paladins are just now starting to become raid viable I'd give you guys a better understanding to how I'm different than a warrior so you can adjust when I'm tanking. I figure tanking breaks down into two categories: Mitigation and Threat. I'll detail both here.
First, the most immediately noticable difference is that I use mana instead of rage. This means that I can go full out from 100% to dead on mobs because I don't need to wait for rage. You'll also notice that I have about 5000 mana with buffs. The only way I sustain a mana pool is from the paladin ability Spiritual Attunement. In a nutshell, spiritual attunement takes 10% of the amount I get healed and returns it to me in mana. The key is, I have to be getting healed by someone else. This means two things: 1) Earth Shield and Prayer of Mending don't give me mana when they heal me (not a huge deal) and 2) If things aren't doing damage to me and I'm not getting hit, I'll run out of mana and be useless.
Mitigation: I mitigate damage the same way a warrior does. I can dodge and parry attacks to completely avoid damage as well as blocking a portion of melee damage. I have a few talents that are different from the typical warrior mitigation talents. I'll list them here.
a) Righteous Fury: When active I take 6% less damage from all sources. Also, this talent makes my holy damage cause 90% more threat than normal.
b) Spell Warding: I take 4% less spell damage from all sources, flat out. It's just a talent and it's sort of a filler talent but hey at least it's here.
c) Improved Holy Shield: Increases my block percentage by 35% for the next 8 attacks. Each time I block with this buff up (10 second cooldown) I lose one charge. This is why paladins are ideal at tanking multiple targets, because I don't take crushing blows because shield block is almost always up. Holy Shield also has a reflective damage component which I'll explain further down.
d) Ardent Defender: This talent is what makes me hard to kill. While I don't have shield wall like a warrior does, I have automatic damage reduction whenever I'm low health. When I am at or below 35% health, I automatically take 30% less damage until I'm brought up to above that number. This means when I get hit hard by a boss it's rare that the next hit can kill me. With my current armor mitigation pre-raid buffs a boss that normally hits 0 armor targets for 10000 hits me for 4000. If I'm at or below 35% health, his 10000 damage attack only hits me for 2800 damage.
e) Weapon Expertise: I'll go into greater detail about this below, but this talent also (or perhaps primarily) affords us 10% extra total stamina. After all, tanking isn't about how much damage you don't take, it's how much you can take.
a) Reflective Damage: A large portion of our threat comes from reflective damage upon being hit. Holy Shield, Retribution Aura and Blessing of Sanctuary all trigger holy damage when you're hit while buffed with them. This is what enables us to 'AoE tank' so effectively. Consecrate is definitely the most visible in terms of our tanking abilities, but it's usefulness pales in comparison to Holy Shield. Damage done from Holy Shield has an additional threat modifier beyond Righteous Fury of 35%. Without blocks, our threat generation could be described as meager at best.
b) Direct Damage: Consecrate, damage from Seals & Judgement and other niche abilities make up the other half of our threat generation, and it too is also almost entirely holy damage.
Last, a word about demons and undead. There's nothing really special about paladins against undead and demons except that it opens us up to use 2 new threat generating abilities: Holy Wrath and Exorcism. Holy Wrath is a 2 second cast which hits all demons around me for ~1000 holy damage. Obviously I can't cast this while tanking because I'm getting hit, but sometimes I can pull with it. Holy Wrath is on a one minute cooldown. The other ability is exorcism. This is an instant cast ability that does 800-900 holy damage to my target, and it's on a 15 second cooldown. This is why on demon/undead mobs (Karazhan, Magtheridon, Hyjal/BT et al.) a paladin's threat output is exceptional. Having an extra heavy damage ability in our rotation makes for spectacular burst threat generation.
