Cooldowns

Cooldowns are great, and are one of a tanks best friends. The simple definition of a cooldown (as far as we're concerned here), is a thing that helps the tank not die.

And we don't like dying, do we?

But beyond that wonderful (and eloquent) explanation, there are several types of cooldowns, and several different considerations to be made when choosing a button to help not become a permanently empty health bar. The net goal may be to help the tank not die, but the route for getting there requires foresight, as different cooldowns give different results.

Flat Damage Reduction Cooldowns

These are the primo-cooldowns. When you press this button, there is no doubt in your mind that you will take less damage.

Warrior Cooldown: Shield Wall
External Versions: Pain Supression (Disc Priest), Hand of Sacrifice (Paladin, when used in conjunction with Divine Shield).

When these are ideal...
-You know a big burst is about to hit, and it's going to either exceed your maximum health in one shot, or do damage at a rate higher then your healers can heal. (ex, Plasma Blast)
-You are picking up a mob unexpectedly, and need to give healers time to react. (ex, current Thorim Tank dies).

When these are weakest...
-When you are critically low on health, and not expecting a heal to land. Taking even 10% of a 20,000 damage hit will still kill you when your at 2000 health.

Maximum Health Increasing Cooldowns

These are great, in the effective health sense. You won't take less damage (heals will still need to be greater then incoming damage) - but in most cases, you'll be able to take an extra hit (or two) when avoidance fails.

Warrior Cooldown: Last Stand
External Versions: Battlemaster's Trinkets

When these are ideal...
-In preparation of spike damage. They have long durations, during which you have a much larger pool of health to fall back on.
-When heals just need another swing's time to land in an emergency.

When these are weakest...
-When damage increases, but healing can't increase to match.
-When heals are not expected to be comming.

Avoidance Increasing Cooldowns

There are no native warrior cooldowns that increase avoidance (they always come from items). They are generally weak, but have a place.

Example Sources: Heart of Iron, Defender's Code, Monarch Crab, Valor Medal.

When these are ideal...
When your trying to give healers a little break, while they are busy (Feared, Raid damage spike).
When your trying to avoid a certain debuff being applied (Ex, Overhead Smash)
When you need to be a little more lucky (Thorim at high charges, where two consecutive hits will simply kill you).
When your already critically wounded, and really really are hoping you dodge the next swing.

When these are weakest...
When you absolutely need something to happen. An 8% avoidance increase guarantees nothing.

Armor Increasing Cooldowns

Armor cooldowns are along the same lines as max health increasing cooldowns, but on a physical-damage only side. While they are more mild in overall effect (compared to last stand, for example), they have the benefit of actually reducing the damage you take - resulting in heals being more meaningful.

Examples: Furnace Stone, Indestructible Potion, Stoneform

When these are ideal...
-In preparation of spike physical damage or soft enrages.
-When healing is compromised, and you need to get further on what your getting.

When these are weakest...
-When your taking magic damage.
-When your already critically low on health.

Health Recovery Cooldowns

Healing isn't a tanks job. We just aren't nearly as good at it as the people with mana bars waving their hands in the back. Health Recovery Cooldowns are the weakest ones out there. While they are more effective in five man content, their effect on ten and twenty-five man raids is minimal.

Warrior Cooldown: Enraged Regenration
Item Examples: Healthstones, Healing Potions

When these are ideal...
Your just under the amount of health needed to get though a potential fatal swing, and not confident a heal is going to land.
Your in the middle of sustained high damage, and need to give heals time to keep landing.

When these are weakest...
When your in the middle of being healed anyway.