Tanking: Formulas and Mechanics
I've spent some time in order to gather data about formulas and mechanics that could be possibly connected with tanking. This post consist all the most important information collected from various websites, forums, other's experience and my own. Hope it will come helpfull one day.

Attributes

Strength
1 Strength grants 2 Attack Power.
1 Strength grants 0.05 Block Value.
1 Attack Power equals 0.07 dps.

Agility
1 Agility grants 2 Armor Class.
1 Agility grants 0.03 Critical Strike.
1 Agility grants 0.034 Dodge.

Defense Skill
1 Defense Skill grants 0.04 Dodge.
1 Defense Skill grants 0.04 Parry.
1 Defense Skill grants 0.04 Block.
1 Defense Skill grants 0.04 less chance to be Hit.
1 Defense Skill grants 0.04 less chance to be Critically Hit.

Weapon Skill
1 Weapon Skill equals 0.1 Critical Strike chance against monsters above your level.
In addition Weapon Skill wokrs quite opposite to Defense Skill.
Your chance to Miss decreases by 0.04%.
Your chance to score a Critical Hit increases by 0.04%.
Your opponent's chance to Block your attack decreases by 0.04%.
Your opponent's chance to Parry your attack decreases by 0.04%.
Your opponent's chance to Dodge your attack decreases by 0.04%.

Combat Attributes

Defense Rating
1 Defense Rating grants 0.4166 Defense Skill.
This means that:
1 Defense Rating grants 0.0166 Dodge.
1 Defense Rating grants 0.0166 Parry.
1 Defense Rating grants 0.0166 Block.
1 Defense Rating grants 0.0166 less chance to be Hit.
1 Defense Rating grants 0.0166 less chance to be Critically Hit.

Weapon Skill Rating
1 Weapon Skill Rating grants 0.2564 Weapon Skill.

Dodge Rating
1 Dodge Rating grants 0.0539 Dodge.

Parry Rating
1 Parry Rating grants 0.0423 Parry.

Block Rating
1 Block Rating grants 0.1266 Block.

Hit Rating
1 Hit Rating grants 0.0633 Hit.

Critical Strike Rating
1 Critical Strike Rating grants 0.0452 Critical Strike.

Resilence Rating
1 Resilence Rating grants 0.0256 less chance of being struck by any type of Critical Strike.
1 Resilence Rating grants 0.0512 less damage taken from any kind of Critical Strikes.

Haste Rating
1 Haste Rating grants 0.0632 Haste.

Additional Note
In order to check relationship between "Factor Rating" and full percentage value please visit http://www.wow-europe.com/en/info/basics/characters.html I decided to put it in a different way because in my opinion it's easier to compare which item improves you the most by multiplying base values.

Rage Generation

Rage Generated By Dealing Damage:
Damage Dealt multiplyed by 7.5 divided by 274.7.

Rage Generated By Taking Damage:
Damage Taken multipled by 2.5 divided by 274.7.

Migitation

Miss
Attack missed. This event is a 5% standard for mobs and players. There is no percentage limit for this factor. Attacks from behind can miss. You can be missed when stunned. This does not generate rage nor threat.

Dodge
Attack is dodged. This event is a 5% standard for mobs and players. There is no percentage limit for this factor. Attacks from behind can be dodged. You cannot dodge when stunned. This does not generate rage nor threat.

Parry
Attack is parried. This event is a 5% standard for mobs and players. There is no percentage limit for this factor. Attacks from behind cannot be parried. You cannot parry when stunned. When parry occurs it reduces the swing timer of your current swing by 40% of weapon speed to a minimum of 20% of the timer. Parry does not generate rage nor threat.

Shield Block
You block the attack. A number equal to your block value is substracted from the attack. This attack is not a crit, nor a crushing blow, for reasoning see below. This event is a 5% standard for mobs and players. Attacks from behind cannot be blocked. You cannot block when stunned. There is no percentage limit for this factor. Block does generate rage and threat depending on the amount of damage taken.

Crushing Blow
Crushing blows hit for 150% damage. Players can never land crushing blows. Only white damage (auto-attacks) can be crushing blows. Spells and specials like Mortal Strike or Mighty Smash cannot be crushing blows. If your defense skill is at your level maximum, then mobs 3 or more levels above your level can land crushing blows with a basic 15% chance to do so. For every point of defense skill you are under 5 times of your level, the chance of a crushing blow increases by 2%. Defense from gear does affect your chance to receive crushing blows unless your base defense skill is lower than 5 times your level.

Armor

Increases your physical damage migitation according to the formula:

Armor M = --------------------------------- A + (467.5 x Mob Level - 22167.5)

Note that armor mitigation has a percentage capacity of 75.

Level Differances

Just as each level that the mob is above you, increases it's chance to crit by 0.2%, it also reduces it's chance to miss by 0.2%, and your chance to dodge by 0.2%, parry by 0.2%, and block by 0.2%.

Every Raid Instance Boss is level 73. That means chance to crit is increased by by 0.6%, it's chance to miss is reduced by 0.6%, and your chance to dodge by 0.6%, parry by 0.6%, and block by 0.6%.

Therefor a main-tank needs 490 defense skill in order to be immune to critical strikes. 5.6 devided by 0.04 equals 140. 350 defense skill is base value for every level 70 player. 350 plus 140 equals 490 defense skill.

Avoidance Comparison After 490 Defense

Defence Rating
1 Defense Rating grants 0.0166 dodge, 0.0166 parry, 0.0166 block, less chance to be hit, 0.0166 less chance to be critically hit. Since at 490 Defence Skill critical strikes from mobs will be wiped off the Attack Table (see below) and the chance to block is a small bit of mitigation, not actual avoidance, so we'll ignore these two. That translates to 0.050 pure avoidance per point of Defense Rating.

Dodge Rating
1 Dodge Rating grants 0.0539 Dodge.

Parry Rating
1 Parry Rating grants 0.0423 Parry.

Conclusion
It seems like Defence Rating is the best Avoidance Attribute. It's nearly as good as Dodge Rating in pure avoidance. In addition it gives a bit of block chance which alwayas comes handy when our enemy hits fast. In PvP it's also reducing chance to score a critical strike on us.