On live servers, when you attack with an MCed mob, and your MCed mob dies, all of the other enemies will immediately aggro onto you. I have seen a few DM solo tribute runs (from pirate servers) where a priest MCs an ogre, aggros the room to kill it quickly, and all of the mobs return to their original positions. Does anybody remember how it worked in Classic, or had the opportunity to test/see this on the beta?
I remember the Priest getting aggro if the MCd mob died.
Aggro transfers to you the goal of the MC is for both to be at either low health or kill one and the other is at low health. Either way the aggro that the monster had is transferred to you.