So I've been running dungeons a lot (totaling over 100 dungeons up to level 44), as tanks, as attacker, and as healer. Here are some common mistakes a lot of tanks are making. Please don't be one of them.
1. Gladiators relying on Riot Blade combo
I've seen a lot of Gladiators who never or very rarely use the Savage Blade combo. They will keep using the Riot Blade combo even when their mana is full. In one extreme case, the Gladiator never used Flash, relying 100% on the Riot Blade combo. After explaining to him why he should always use the Savage Blade combo unless he needs mana, he changed into a sort of 3 Riot to 1 Savage ratio kind of rotation.
While some of you may think that Riot Blade combo has higher potency and hence more enmity, this is completely wrong. The Savage Blade combo is designed specifically to produce much more threat than the damage it does. This combo is your main weapon. Do not use Riot Blade unless you don't need the added enmity.
And you will need that additional enmity if the attackers in your party are worth anything. Getting away with only Riot Blade and still never have targets turning towards the attackers would mean your attackers need to relearn their class, too---or the opposite and they're holding off and doing nothing half the time waiting for you to generate enough enmity for their next attack.
2. One flash/overpower per fight
While you could get away with one Flash in the whole fight during your class quest battle, that was because the battle was designed to be that way. Flash is no longer the strong enmity tool it used to be back in beta. Now healers heal more, and overheal more. Healing causes much higher enmity now than it used to.
The right way is to always keep switching targets and put down a combo'ed Savage Blade on a target that is racking up healing enmity. If you can't be bothered to switch targets, then make sure you cast plenty of Flash if the pack consists of more than two enemies.
3. Tanking with your backside
Don't let enemies hit you on your back. Move and adjust your position so that all of them are in front of you. You can't parry or block if attacks are coming from behind you. You are making things harder for your healer. You are endangering the whole party by doing this.
4. Continuously shifting around the target
I don't know where this one came from, perhaps from the urban legend that, "just keep going in circles around the mob and it can't hit you because it has to turn first."
That rumor is not true. Mobs now are able to instantly adjust their facing after using a weapon skill, and auto-attack is no longer tied to any animation, so as long as you are in melee range, you will take that hit even if you are rotating around the mob at 10,000 turns a second.
Not only will you still take the same hits, you are also making it close to impossible for Pugilist and Lancers to deal proper damage. They have many attacks that need to land on either the side or the back of the enemy. Shifting around like that means they deal less damage, fights last longer, and in the end you take more damage.
Good tanks know to minimize movement. Don't move unless there's a good reason.
5. Casting Protect
Do not cast Protect if you have a Conjurer/White Mage in the party. Conjurers have a trait that adds magical defense to their Protect. You don't.
So stop casting Protect!
6. Leeeeeeeeeroy!
Tanks often take the lead, but a lot of tanks these days are just rushing forward to the next pack and delve right in. Don't do this.
You need to check on your party members all the time. Keep them in sight. Wait for them. Especially the healer. Good tanks will keep a brisk pace and move ahead, leading the party. But at the same time they know exactly where everybody are. They always keep an eye on the whole party even if the party never realize it.
Turn around. And do it often. A tank isn't a slave master with the rest of the party on a leash. A tank is the protector of the herd. You stand in front because you elect to stop all harm. You can't do that if everyone else is a whole room away and trapped behind an obstacle (which you triggered).
※ Leeeeeeeeroy!