Figured I should add some more to this because Paladins are very good at holding things down the the stuff hits the fan. I got motivated to write in here again after playing with the LK for a couple hours last night and getting him to 39%(another hour or two and I know that group could have downed him). I'll go through some scenarios with bosses and trash dealing with ICC as this is where most tanks will see these issues.
First thing to do on trash is to hopefully figure out which trash casts and which trash melees.
Obviously if you are going to be pulling a group throw your shield at the casters to silence them and use your hammer of righteousness when the melee get to you.
Multiple casters? - User arcane torrent and shift to the other casters, the ones you were already tanking from the shield will be interrupted and move with you.
Now the fun part.
Extra adds are fun, if controlled everyone feels like they had a great time overcoming the obstacle instead of being bored by pulling trash, of course if you get too many adds and everyone dies that takes even longer plus you just added to everyone's repair bill, congrats!
There are a few types of reactions to extra pulls I use and/or see.
The wrong way
A lot of this is dependent on who is tanking with you.
A warrior? Net or Protect work the best.
Another pally? Any of these would work.
A DK? Definitely net or protect.
Druid? Charge or protect.
These change depending on caster versus melee, but the general idea is this.
Simply put, if you are between the raid and the new adds, the net is to lay down all your AoE in the way of the new mobs, using taunts and shield throws, maybe Holy Wrath if they are undead.(awesome for those traps that spawn a ton of leaper dudes in the plagueworks.) The idea is to keep all the old adds on you and let the new ones gather into your spot. If the raid is paying attention, at the words, "New adds!" the rogues and hunters will be using tricks or MD to help you out anyways, + that 15% extra damage from tricks helps you keep threat longer.
I recommend tab or click targeting while doing this to make sure you are building threat on the creatures with the most life so they are not pulled off of you as easily.
This is always used by me no matter which other method I use, which is why protect is mentioned in every variation above.
Protect the people that keep you tanking! When I hear, "Extra adds" or "More spawns" there are two things I look to do.
1. Use healbot to click multi-taunt adds off of any healer that currently shows having aggro.
2. Use hand of sanctuary on any dps/or another healer that looks like they are pulling aggro or already have aggro. (easier to pull off of them if they aren't gaining threat + they get to keep healing the entire raid)
3. Use Hand of Protection on a healer, make sure at least one healer is up to heal the raid! and all those mobs aggro'd to them can come get tanked instead.
4. Use divine sacrifice with or without a 50% cooldown, either way you are saving the other tanks and anyone else in your group from some damage.
Note: I try to always pick a druid or a shaman to use these abilities on for a few reasons.
1. Paladins have armor and their own bubbles, they can beacon themselves, they know how to survive if not they should figure it out.
2. Disc priests have a cooldown as well and spirit priests have spirit form which is pretty useful to keep the raid up while gathering up the adds.
3. Druids and Shaman are awesome raid healers, but horrible at keeping themselves alive! Keep them alive and they can hot the whole raid or use that OP T10 4 piece shaman set to heal half the raid themselves.
Sometimes the best thing to do is to just run straight at the new mobs, hit them with a few attacks while running through them and then turn around and backup to catch all the mobs following you. Wallah new area to fight, this is most helpful with cleaving mobs or new casting mobs as it is better to move the battlefield to them rather than risk casters aggro'ing to other casters or healers or melee getting cleaved by the new adds running in.
The wrong way
Stand and do nothing, attack only the mobs you currently have, run for the exit, pull all the mobs back to the healers, DI a healer assuming the worst, start screaming "game over man game over!" in vent, when you die sitting around waiting for the fight to end to get a res. Release and run back fool!
1. Everything that can go wrong will go wrong, get used to it.
2. Tanks should expect healers and DPS to never give up on a fight, just assume they will do everything they can to win.
3. The worst that could happen is now both tanks are dead and the fight is over usually at this point anyways, at least you tried!
Examples of things gone wrong, but ending fine
Boss fights are mostly about knowing the mechanics, once you know what you are supposed to do as the tank, figure out what everyone else has to do, that way when everything goes wrong you can try to help keep things going as best as they can.
I've tanked Anub 25 man as ret for phase 3. I was playing ret to let the other tanks get experience tanking him and they died so I taunted and proceeded to tank him with ranged attacks and running around in circles forcing him through ice patches that slow him down so he couldn't ever hit me with anything. He died, we won, no time wasted on flying back, rebuffing, etc.
I've tanked 2 Large Oozes and Rotface 25 man at the same time. Boss tank died to an extra large ooze showing up under the boss(it happens all the time) so I taunt the boss and the adds then use strafe running to run around the room keeping aggro on the boss. The slimes could not catch me, I kept threat on the boss, DPS and heals kept right on going and we killed the boss.
Single Tanked 25 man Festergut after other tank died on 3rd inhale. Taunted boss, smashed my 100% bubble /cancel aura macro to clear off my 9 stacks of bloat so I didn't explode and tanked the boss until done. See I told you that macro rocks!
Heroic 10m Anub: there is a video of this, boss tank died, I ended up tanking Anub + 2 adds for a bit , then died, then the boss missed burstorc(enhance shaman) with an attack and we won.
Dethbane actually tanked 2 beetles and KT way back in the day after I ate a void zone and we won.
Raynen(rogue) has tanked more than a few bosses from 3% to dead, the healers know he's gonna attack and try evasion if we are close and the tanks die, so they heal him.
Plate DPs have tanked bosses until the end of a fight, the healers never quit unless yelled at, even then they do not always stop healing.
How to adjust on boss fights
Simple, if it hits the fan, pick it up.
Notice a trend in the stories above? If you have a taunt, and an ability to absorb damage, you can help. Hunters can taunt and you can taunt back to limit some damage on yourself, other plate or rogues can tank stuff until you get the rest of the crap gathered up.
Sometimes all you need to do is taunt everything in the entire room until they battle rez the other tank or the boss/adds die.
Be prepared to have to kite if you pull that many mobs, making it take longer for them to get to you after taunting saves you valuable time. On Icehowl I'd stand far away from the boss when he slams into a wall because if he enrages chances are the tank with aggro right next to him is getting killed, I need to be able to taunt and give the most time possible for the enrage to wear out before it gets to me.
If you are tank #3 and Deth and I are tanking, you have to know what each of us does so if either falls you can pick up the slack. This way later we can ask you to do either and you will know how.