Part of the problem here is that certain classes/specs have no aggro dump or reduction ability and as a result their big DPS moves generate a lot of threat. Arms warriors, most DPS death knights (at least the blood and unholy specs I've used) and retribution paladins have limited aggro reduction at best (pallies at least have Hand of Salvation but that's a band aid at best) and while tanking threat (especially AoE tanking threat) requires ramp up time, it's very easy to hit the Divine Storm, Death and Decay, Pestilence, Whirlwind or Bladestorm buttons. (Enhancement shamans with Spirit Weapons seem to be okay, I rarely pull aggro even when I go nuts with Fire Nova.)
I won't pretend I'm not guilty of this myself when I go DPS. In general this seems to be an issue with the plate classes, as they're designed to be able to tank and thus have not traditionally been designed to shed aggro so much as to generate it. It also doesn't help that since they're in melee range it takes less to pull aggro. So, as a DPS'er, how can we be friendlier to the tank with our big AoE abilities? Here's a few quick and dirty suggestions.
- Wait for the tank to stop moving. This isn't always easy to do, but for big AoE pulls the tank often has to gather up mobs that are not close together and get them all close enough for his AoE threat move to work. Running up to a mob and hitting your big AoE while the tank is heading over to another mob not only wastes the ability's AoE damage, it almost certainly gets you aggro.
- Wait for the tank to use an AoE threat ability. If the druid hasn't Swiped, the DK hasn't hit Blood Boil or put down D&D, there's been no Consecraton from the paladin or the warrior hasn't used Thunder Clap, Shockwave or Cleave, it's not safe to use Bladestorm or a similar ability.
- Make sure you're in the same area as the tank's AoE threat. A good example is the trash pulls before Scourgelord Tyrannus in Pit of Saron. If the tank has four mobs in close proximity to him and one off in the distance hurling spells at him, don't Divine Storm on the one off in the distance casting spells. If there are two casters and five melee and the tank has not really bothered to try and hold the melee yet because he's Death Grip pulling the other caster to him, don't hit Bladestorm yet. Hold off on AoE until you see the tank has his AoE hitting the mobs you'll hit.