I would say its going to be based on Healing / Damage absorbed/done in some weird complex calculation, with a big include of previous contributution..
I read a post on the Warhammer-alliance forums implying that you get more credit for healing the person who takes the most damage... Also that the player who absorbs the most damage is rewarded... So is the person who damages the PQ targets the most... (*I will dig this out and post it*)
That may explain why as a Warrior-Priest I always do pretty well, because i'm constantly healing the main tank and hardly anyone else.. I'm also DPSing the main targets and I can tank most of the lesser mobs.. So I win-win-win it seems!
After repeating a PQ 3 times, I was winning every single PQ due to my previous contributution....
However I have never got anything more than a "Lesser Loot Bag" in any PQ upto Tier-3, and that is after winning by a large margin!
Dont know if that was on any help
Ta,
Invicta