I made a little chart weighing the strengths and weaknesses of the four classes in several catagories:
- Damage Mitigation: The more damage you can avoid taking, the better it is for your healers, and you will stay alive longer. This is typically melee damage. Paladin's typically take the least damage overall with blocks and high armor. Druids and Warriors take similar amounts of incoming damage depending on the uptime of a druid's Savage Defense; Druids have higher armor, so I would assume their damage intake will be lower. DK's are made to be avoidance tanks, so they will get hit harder when attacks land as they typically have less armor than the other classes.
- Total Health: The more health you have, the bigger hits you can take before you die. This stat is most useful for bosses who deal large blows very slowly. After 3.1 DK's and Druids got big HP nerfs but they still have the largest health pools of all the tanks. Paladins are next since their stam coefficient is higher than Warriors.
- Avoidance: How often the boss Misses/Dodges/Parrys when he tries to attack you. Avoidance is typically a secondary category to gear for as it is based on random chance. If you fail to avoid several attacks in a row, this could end up killing you if you cannot mitigate the damage or if the incoming damage is more than your total HP. DK's win this round, followed by Druids with their crazy high dodge rate. Paladin's typically have more avoidance than warriors ceteris paribus as agility gives us more dodge than warriors, although I don't see much plate with agility on it. I need to get some data for the warrior/pally avoidance.
- Cooldowns: If you can use cooldowns to prevent/mitigate a large incoming attack, the less likely you are going to die. The more cooldowns you can use, the better your chances of surviving. Warriors have some of everything including their buffed shield block. DK's have short cooldown but potent abilities, which edges them out against warriors. Druids have a couple tricks. Paladins have one talent, with one glyphed not as good cooldown (Hand of Salvation).
- AOE Tanking: Sometime or another you will need to tank more than one angry at a time. This may be 2, or may be 20. The less GCD's you need to use to keep them under control, and not get eaten by them, the better. Paladins shine in this category as we take less damage from many angrys beating on us while maintaining threat. DKs have very good/best AoE threat, but take more damage due to being avoidance tanks with not as much armor. Druids have swipe spam, and can take less damage with higher armor. Warriors have a lot more going for them in terms of AoE tanking than they used to including Damage Shield, Shockwave(Long CD), TClap, as well as Deep Wounds on crits (Thanks Warriors for the info)
- Threat: The tank needs to be the most threatening person in the raid to an angry. If you cannot keep more threat than the DPS, they will be taking hits instead of you. Paladins have the highest threat due to our threat abilities being magical in nature and not mitigated with boss armor. DKs have some magic damage, Druid/Warriors bleed which is reduced by armor.
- Moblility: A tank needs to be able to intercept incoming new angrys as well as get away from bosses when they become too dangerous to tank. The ranking is very tough as each class excells in a various movement type. Warriors take this one as they can charge angrys and friendly players. Druids have have the ability to charge. Paladins and DKs have passive run speed talents; DKs can also bring angrys to them instead of intercepting them.
- Raid Assistance/Buffs: Some tanks bring raid buffs and can cast spells to assist your raid in completing your tasks better. Paladins edge out warriors as we have Hands of XYZ, Divine Sacrifice, and BoSanc. Warriors can Intervene, Sunder Armor, Vigilance, and have a few more tricks to help out the raid (Shouts etc). Druids have a buff (MotW, Leader of the Pack), and DKs do very little to provide additional raid support.