So, looting goes like this (assuming there are two players/parties):
Loot goes to the player that dealt the most damage to the mob. So silver/lootbag or the lootchest if it was a dungeon boss goes to the player that dealt 51%+ damage to the mob.
If both players are not flagged for PvP, then the loot has a 2 minute loot protection, to ensure it goes to the player that earned the kill.
When the player that did not get the kill is flagged for PvP, they can circumvent the loot protection though. That means, in the black zone, there is no loot protection as everyone is automatically flagged for PvP.
Having the aggro during the death of the mob does not automatically give you the loot, if you didn't to the most damage to the mob.So if someone in plate comes with the intention to take the boss there is nothing you could do except killing him
If you have documented situations that contradict the current loot mechanic, feel free to let us know.^^