There was an extremely good post about threat and aggro on the WoW forums a couple days ago. Unfortunately, I can't find the post now, which is a real pity.
Update: Gitr found the post that I was referencing. Comments below have been corrected.
The poster undertook a series of experiments to quantify "threat" in WoW, and the post detailed his results. The main conclusions of his experiments were as follows:
- Threat is cumulative: You accumulate threat throughout the entire fight. The longer the fight, the higher the threat totals are.
- 10% Barrier:To take aggro from someone, you must do 10% more threat than them. For example, if a mage has 100 threat, you need to do 110 threat before the mob switches to attack you. This means that the person with aggro may not be the person with the highest threat level. The barrier increases to 30% for a character at range.
- Taunt: In addition to forcing the mob to attack you, taunt is a "threat-shortcut". When you taunt, it permanently gives you the threat level of the person with aggro. It also recalculates aggro for the mob, most likely giving the warrior aggro permanently.
In particular, the way Taunt works is interesting, especially on long fights. Taunt is essential for pulling a mob off someone, because it gives you a huge gain in threat in a very short time. However, paladins don't have a taunt, making it very hard for them to take over tanking a mob.
For example, consider a druid, priest, and off-tank warrior. The druid is tanking and generates 2000 threat, the priest is healing and has 1000 threat. The OT warrior has 250. The druid gets crit hard and dies. The mob attacks the priest. The OT taunts, and now has a threat level of 1000 and aggro from the mob.
Now consider the same scenario with a paladin as off-tank. The mob attacks the priest. The paladin is at 250 threat, and needs to get 850 more before the mob turns away from the priest. Odds are the priest is going down.
(Of course, a paladin would probably toss Blessing of Protection on the priest, causing the mob to turn it's attention to the paladin and giving the priest and paladin 10s to fix the situation. But BoP has a 5min cooldown, making it a bit unreliable.)
So a paladin needs to get as much threat on a mob as possible, because she lacks the instant threat gain of Taunt. Because paladin healing is fairly low-threat, the more a paladin heals, the worse her position becomes if she is suddenly called on to tank. Of course, the more a paladin heals, the less likely it is that the warrior will die and she need to tank. Trade-offs are what make life interesting.