Article source: Blizzard Warcraft Forums
Author: Rake

1) Definitions

We define "aggro" to be who the mob is attacking. We define "threat" to be a numeric value that each mob has towards each player on it's hate list. Note, as we shall soon see, even for a normal mob, the target who has aggro is not necessarily the player on it's threat list with the most threat.

We define arbitratily that 1 point of unmodified damage gives 1 point of threat.

2) How to gain aggro - the 10% barrier

Simply put, for a mob to change aggro to a new target, the new target must have over 10% more threat than the mob's current target. E.g. mob is attacking player x. x does 100 damage to mob, then stops. Player y starts hitting the mob. The mob will start attacking y when y does over 110 damage.

Proof: this is easy to demonstrate. Get two players both doing autoattack on a mob (not warriors of rogues; we'll see later they complicate things). Have player 1 do a certain amount of damage, then stop. Have player 2 keep attacking till he gets aggro. You have an upper and lower bound on the threat required to get aggro - 1 attack before he got aggro was not enough, but the attack that he got aggro was at least enough. With low damage attacks (i.e. fists only), you will get a very good value of 10%.

This is only a description of the normal mob targetting. Obviously there are mobs who will attack secondary targets with special abilities, ignoring their current threat / aggro.

3) Threat modifiers from Warrior Stances

In Battle Stance and Berserker Stance, all threat from a Warrior is multiplied by 80%. In defensive stance, the multiplier is 130%. With Defiance, it is 145%.

Proof: a simple modification of the above proof. Get a warrior to do, say, 1000 damage in defensive stance, without defiance. Get a non-warrior to take aggro with white damage. You will find it does not happen before 1430 damage. The warrior's 1000 damage caused 1300 threat in defensive stance, and the 10% barrier means you need more than 1430 to gain aggro.