The common mistakes

[Jaminos]

Tunnel Vision
Tanking Tunnel vision has killed more players than I care to count. Don't get so focused on the boss that you ignore everyone around you. What I do is scroll out as far as I possibly can with the camera so I can see the entire battlefield. You should be split about 60/40. 60% on what's going on around you and 40% on the boss. HOWEVER this does not mean you should ignore the boss by any means. Pay attention to what abilities he is using and the AOE's he is casting. As well as agro always being your top priority.

Face the boss
A good rule of thumb for most fights is to simply face the boss away from the party. This should be an automatic function for you. It is for me at least. You want the boss facing you at all times and not the party because many bosses have frontal cleaves and frontal AOE's. Most bosses will have rear attacks that will hit your party, but all the agro in the world won't help you. It is a forced attack that usually but not always, can't be stopped.

Stun
Not every tank can stun right away. But if you have it, use it often. If you see a slower cast, stun it! Interrupting a slow cast will save from heavy damage on you or your party (Or sometimes both)

AOE attacks
This is a very obvious mechanic of almost every boss. When you see the red AOE pop up, GET OUT OF THERE! Don't just sit there like a deer in the headlights. You will take massive damage or sometimes even a guaranteed death. Some bosses like Titan will confuse this mechanic where being within the AOE is a good thing as long as you are not in the center of it.

Hubris
Don't get cocky! You are not a tanking god! Just because you have awesome armor and weapons does not make you a great tank. Its better to just keep quiet while tanking, unless you need to explain something or say something relevant to the situation at hand. The moment you start acting like you are the most important player on that battlefield, players will drop out. Its ok to have a good time and be friendly. But never overestimate your worth. Everyday a new tank is made. Others will gladly take your spot. Just because higher level tanks are rarer does not mean you are not expendable. I rather fail a dungeon than put up with a tank who makes an ass out of himself.

Know your rotation
Before you start dungeon, always make sure you know what skills do what. If you level up in the middle of a dungeon and get a new skill, spend a moment to read what it does and place it in the appropriate spot in your rotation. You would be surprised how many lower level tanks just spam random abilities, not know what they do.

Taunt
The very most important skill in our arsenal. While I am not as familiar with MAR/WAR tanking, Gladiator/Paladin's all get fairly early on "Flash" our AOE taunt. Use it, use it often. If you see a member of your party have an NPC break off to attack them, grab it quickly. Many tanks fail to use their taunt to its maximum effectiveness. You should be taunting the enemy as much as possible. Just make sure you don't use up your MP because you spammed it TO much.usually your three hit combo done twice is a good time to use Flash if you have it.

Buffs
I always grind my teeth when I see a tank who does not wait for buffs. This is such a simple thing to wait for. Also, please wait for EVERYONE to get their buffs. Don't be a jerk and just rush ahead after you get your buffs. Waiting for the healer to have their MP recover after buffing is also another good policy to have. You will wipe during some boss fights. It happens. Don't charge the boss after your buffs are applied while the healer is still recovering their MP. In the end all you are doing is hurting yourself because it means that's just less MP they can use to heal you during the boss fight.

Gear
I never watch videos of a dungeon before I go into it. I love the surprise and figuring out each boss. HOWEVER, I do look up the recommended gear requirements for every dungeon. After Manor, dungeons in the level 30± range jump up drastically in difficulty. Gear becomes an important corner stone to any good tank. Make sure your gear is always at least within four to five levels of the dungeon. Anything less and the NPC's will take you down before you knew what hit ya.

Also, pay attention to what you are rolling on during boss loot. Just because you can use it, does not mean its for you. Sometimes other DOW gear drops and while you can wear it, leave it for other DOW classes unless it is such an important upgrade that you HAVE to roll on it. But usually this means you are under geared as mentioned above.

[HEC]

Buffs
I always grind my teeth when I see a tank who does not wait for buffs. This is such a simple thing to wait for. Also, please wait for EVERYONE to get their buffs. Don't be a jerk and just rush ahead after you get your buffs. Waiting for the healer to have their MP recover after buffing is also another good policy to have. You will wipe during some boss fights. It happens. Don't charge the boss after your buffs are applied while the healer is still recovering their MP. In the end all you are doing is hurting yourself because it means that's just less MP they can use to heal you during the boss fight.

^^^^^ THIS!

It makes my blod to boil when I see tank (or any othe class / party member) to ignore this rule ...

What I do now is to buff tank as last and then say "Ready" or just "R" in the party chat. This will also keep my target / selection on tank to be able to heal right away.