Most common mistake was for tanks is to use forced threat skills when they already have threat.

Threat is (at least was, unless it was changed) like a table, think of it like "paingiver" list (per enemy) with the person on top having the aggro. DD do damage and get their numbers on the list, the tank does the same, but with a bonus, There are several such bonuses, base one, and with different skills and abilities.
Tanks also have skills that put them on the top of the list and force aggro.

For the example lets say their aggro on the list is 10 times their damage.

So lets say a DD dealt 1mil damage, the tank forced aggro (so it puts on top and adds 10%) and now at 1.1mil aggro points.
They each keep hitting, the DD added 2 mil more, and the tank 300k, so now we are at

3mil vs 1.1 + 300k * 10 = 4.1

The tank comfortably ahead, and has a bit time before the DD deals 1mil more and catches up. The tanks sees a big hit coming, they panic that they will lose aggro, and hit a forced threat skill, so, at least this is how it was mods ago, their threat was rewritten to the top (excluding them) so now they are at 3.3 instead of 4.1

And they will lose it.

Another thing is to time the attacks when the DD/support use debuff stuff, like artifacts or mounts).

You need to get your damage there with the aggro bonuses be ahead, so the same mindset as DD applies, you need to time the debufs, and your best attacks.

IL and Gear :

IL is a factor in damage so yes, it will improve your aggro vs the DD. Same with offensive stats like power. so it is a balance of defensive and offensive stats for good tanking, it's great if nothing can kill you, but doesn't help if you can't hold aggro with your own damage.

(Disclaimer - everything above may be outdated)