I can't believe I have to write this but a string of bad dungeon runs have convinced me that it is needed.
How to be a good tank
Five simple rules for being one of the best tanks in the game.
1. Stance. Without it, you aren't tanking. You will not keep enmity off the healer or the dps because your base damage simply isn't enough to counter either healing enmity or the damage that a DPS can pour out.
2. AOEs. As long as there is more than one enemy on the field, hit those AOEs like you are getting paid for every time you hit the button. I don't care if your single target attacks do more damage, they only do more damage to one enemy. If there are three or more enemies, your AOEs will do more total damage (take all the damage you are doing at once and add it together - doing 100 damage to three enemies or 300 total damage is always better than doing 200 damage to just one enemy.) But even if there are only two enemies, stay on that AOE. If you aren't doing damage to an enemy you aren't keeping the enmity on it.
3. Don't pull more than you can survive. Dying slows the run down. Dying repeatedly slows it down a LOT. Pulling fewer groups lets your healer help with DPS if he's not having to keep you alive with every spell he casts. If you notice your health dropping fast, you are probably pulling more than the healer and your own mits can keep up with. Try pulling one or two fewer groups until the healer can comfortably keep you at around half health or so.
4. Stay out of AOEs and if there are repeating stationary AOEs like the crystals in Dzemael Darkhold, pull the critters away from them. The less work your healer has to do to keep you alive, the better. The last thing you want to see is all your mits on cooldown and the healer out of mp just because you won't work the mechanics. And letting the enemies stay inside the range of stationary AOEs means that the healer has to split his focus between keeping you alive and keeping the melee DPS alive. Less work for the healer = better run all around.
5. Rotate your mitigations. You can't keep one up all the time, but try to keep one up as much as possible during combat. Again, the less work the healer has to do to keep you alive, the better for everyone. If the healer actually has a chance to help DPS a little, even better. Remember, the healer casting damage spells is the same thing as the healer casting heals or shields, it is preventing future damage by ensuring the enemies die faster so they hit you less.
Yes, I know I left out interrupts and stuns and when to use them, when to use shirk to help swap aggro to the secondary tank, learning the specifics on individual fights, and the rare exceptions to the above rules. All of that is Tanking 201, the advanced class. Using these simple rules will make you a better tank than around 90% of the tanks I have encountered. Learning the advanced stuff will make you a better tank than 90% of the remaining 10%.