Base threat, and why it's good for you.

[Freehugs]

Just figured out something new (to me anyway) and interesting which is good to know.

Enemies that spawn during combat (or at least during boss fights) are spawned with a base amount of threat (varies per enemy). Here's how it looks in the combat logs, with the important bits bold and underlined:
[@Mercdeeps] [Trandoshan Trenchcutter {2876507101855744}] [ {2300869815042048}] [Event {836045448945472}: ModifyThreat {836045448945483}] () <100>

...but tanks get a threat bonus from their stance! Here's how it looks for them:
[@Tankassin] [Trandoshan Trenchcutter {2876507101855744}] [ {2300869815042048}] [Event {836045448945472}: ModifyThreat {836045448945483}] () <200>

So tanks start with double the initial threat, enough in this case that a healer would have to heal 1821 to pull agro from that initial threat (7 enemies in combat, half threat from healing, 30% agro buffer). Single opponents like Warlord Kephess can spawn with 5000 threat, that's 10000 for anyone in tank stance and another 3000 if that tank opens with a Taunt.
Even with out the taunt, a healer would have to put out 18,001 healing to pull agro with just this starting threat.

This does not apply to enemies that are present at the start of a fight.

[Rystet]

Very very interesting.

I had a feeling that some sort of mechanic like this was at work. There are a few encounters that spwan adds and I have noticed this affect, but was never sure exactly what caused it.