Nightmare and Beyond

[Kevin]

As far as aggro goes, I was led to believe they target the closest person and that there's no real aggro percentage. I'm not 100% sure but that is what I've been told by other players.

That seems to be my experience from playing a witch doctor. I can AoE some stuff at range and it will come at me, a barbarian charges in afterwards and they stop coming for me. My minions would cause the same effect. I think the one caveat is that you need to do damage (or heal) to get noticed or at least with the minions. Just getting a zombie dog or gargantuan closer to the mob via a recast doesn't seem to change them over to it until they have hit it.

So I think it might be as simple as "aggro is held by the nearest target that has done damage." There are of course some randomly targeted secondary attacks on bosses it seems (the skeleton king charge for example), and random target AoE attacks or "floor efffects" which should be familiar to any former WoW player and were around in D2 as well.

So ranged mobs will often attack ranged toons that may have initiated combat because the melee range toons AoE didn't hit them.

There may be some sort of aggro threshold but I haven't observed one in game yet.