Honors, over at Honor's Code has begun to notice a disturbing trend. The DPS are begining to catch up to him in threat. This is mostly because their itemization is more focused into the field of damage dealing, this means that as gear level increases, the DPS's damage output increases more rapidly than the tank's does. That's because we have more important things to focus on, like not getting one shotted by the angry crypt lord. The good news is that this means that your DPSers are doing more damage, the bad news is that this is only going to get worse.
Honors found that the two worst culprits are a destro lock and a hunter. The lock is the worse of the two, however, Honors is baffled because he knows the lock's a good player. Well,the reason why locks, and Destro locks in particular create aggro issues for tanks is oddball design on blizzard's part. Destro Locks have a talent called destructive reach. It decreases the threat of their destruction spells by 10%. This does not reduce their threat on their Aff or Demo spells. They can go into the affliction tree and get imp drain soul to get a 10% reduction in threat on their aff spells. This means that at any given time, a warlock's threat is being reduced by, at most, 10%.
Death Knights get a 25% reduction on all attacks from subterfuge.
Moonkins get a 25% reduction on all attacks from Nature's Reach.
Ferals get a 30% reduction just for being in cat form.
Mages get a 40% reduction on arcane spells from arcane subtlty, and a 10% reduction on fire spells from burning soul.
Ret Paladins get a 30% reduction on all attacks from Fanaticism.
Shadow priests get a 25% reduction on all shadow damage from shadow affinity.
Rogues get a 30% reduction on all attacks for being a rogue. Plus tricks.
Ele shamans get a 30% threat reduction on all spells from elemental precision.
Enhance Shamans get 30% reduction on all attacks from spirit weapons.
Warriors get a 20% reduction on all attacks when not in defensive stance, and fury warriors get an additional 10% from improved beserker stance.
The only class with less threat reduction than a lock is... a hunter. However, both classes come with tools to allow them to manage aggro. Hunter, with misdirection and feign death should never pull aggro. Warlocks have soul shatter. Their primary threat reduction ability is on a 3 minute cooldown. Their secondary threat reduction ability is death. Naturally, we don't want them to use the second one, so careful management of soulshatter is important. Along with that comes careful management of threat. Warlocks aren't like hunters, where they can just go, "Oh shit, feign death!" A warlock who pulls aggro is very quickly a dead warlock.
On some fights, such as hodir, the second half of Destructive Reach comes into play. If the boss is tauntable, a warlock might set up with the tank a plan where the warlock actually pulls aggro, the tank quickly taunts back, and the lock then shatters. That tank gets a threat boost, and the warlock gets breathing room. The other method is to methodically ride the tank's threat. I had a raiding warlock who was pro at this, and it freaked me out the first few times I raided with him. This takes advantage of the mechanics of threat. A warlock at range must hit 130% of the current aggro target's threat in order to pull. So while you see a lock blowing past you on omen, their threat meter shows them happily sitting at 85% of the threat needed to pull. Although on Hodir in particular, you get some hilarious moments where fight mechanics force the lock into melee range of the boss, where only 110% threat will pull, and the boss promptly kills them.
In the end, the key to working with a destro lock is communication. In order to maximize their DPS, they will likely be putting out more damage than you can put out in threat. Both of you need to know if there's a hand of salvation or soulshatter available. Both of you need to know if the boss is tauntable, and if the fight mechanics might force the lock into melee range. By being one step ahead of the boss, the lock can still put out killer DPS and not take an impale to the face.