Ok... to give some clarity on why **certain** hunters suffer so badly on threat.
A MM hunter with ArP items and the correct items in his kit, will pretty much sit and feed the tank.
Lemme explain. Cynicster hit the nail on the head with his post.
If a undergeared tank is holding aggro on a 8/9k dps hunter, its purely because the hunter is wasting his GCD's, timing MD'ing shots to the tank, holding back on nuking, and spamming FD at every possible cooldown. Hunters are like cars rolling down a hill... you have to get the rotations flowing and get into the cycle of nuking, build up speed and keep it there... to stop your cycle and backoff on nuking, because his threat is climbing to aggressively, is a serious dps loss. Once your dots are running and you need to know when to refresh them without breaking your timing to MD to the tank, means you pretty much start over on your rotations.
Now hunters with Grim toll or Mjorn runestone will know exactly what I am talking about. Those two trinkets proc so easy, its usually within the first opening shot or second opening shot that the thing goes. Now, once that thing runs, I am on 1400 Arp which is 100% armor ignore. Every single autoshot / physical damage shot like aimed shot, hits like a truck on steroids.
The first few seconds in a pull is a NIGHTMARE for any MM hunter.
Multishot is another evil necessity... you have to use it as a MM arp hunter.
I know you guys say pulling aggro is no excuse, but honestly its hard if the tank is not very well geared up, to aid his threat generation. 99% of the time you will see the tanks are not using the proper threat enchants, since they are pretty confident they can keep aggro.