I'd like to add a few things on warriors as well.
Having been one of the few warriors that has played all the roles that Darian mentioned more than once in both MC and Onyxia I figure I can give a few tips. I'll start on the other end of his list. This is for MC/Ony as well.
Gear: I've taken the other route and use wardrobe to specialize my outfits for various encounters. Fire lords do pure fire damage so I pump that on them. I have high arcane outfits for Shazz, I have a mix of fire resist and def for ancient core hounds. I have a higher DPS set up. I have a DPS with a chance that I'll have to offtank set-up. You don't have to do this and keep in mind if you do you need to be quick with the changes since you can't change in combat. The other thing to keep in mind though is that stamina can almost never do you wrong. Sure optimal may be more def or more FR but when in doubt stamina with decent armor can't hurt you.
Main Tank : Darian covered it. You will have rage flowing in at a huge rate. So you can actually throw out shield slams and heroic strikes as well. These are high aggro skills and with as much rage as you have you don't need to worry about the most aggro/rage (which is actually revenge followed by shield bash). The job is all about aggro.
Main Assist : What Darian said again. When you learn the tanks better you will learn how soon you can call DPS but then again calling at 3 sunders vs 5 saves the raid all of about 10 seconds so the 5 sunder rule is generally pretty safe. As mentioned in places like Onyxia there will be someone who is calling DPS but the raid isn't really assisting them. As mentioned the MA won't always be a warrior either but we like to use them simply because of the CTRA tank windows it makes it easier. We do expect all warriors to understand when and how to call DPS in Molten Core though so pay attention or ask the other warriors when they are doing it.
Secondary Tanks : One skill that you should use if you aren't being beat on is thunderclap. 10% slower attacks on the mob is a good thing.
Some more specifics. We use 2 tanks per Molten Giants to keep them in place if they punt the other tank. The secondary tanks job is to stay just behind the primary tanks aggro on that mob. If you are backing up the main tank generally this means you sit in battle stance, make sure a thunder clap is on the mob and whack it. If the mob punts the MT flop into defensive stance to lower the damage of the one or two hits you'll take if you want. If the MT dies be ready to play tank. Since this is the second target to die you'll have plenty of time to build secondary aggro there is no need to try and take aggro from the other tank the healers aren't looking at your health as closely. If you are backing up the tank on the first target to die you are doing basically the same thing. After they land 5 sunders and DPS is called you may want to throw a sunder or higher aggro hit of your own in there now and then since the raid damage is coming and your job is to make sure that thing hits you when it punts the other tank.
If we have the tanks we'll also assign you as a back up in fights like Lucifron and Gehennas on the guards. In the Luci fight especially with the mind control you have to be on your toes. When we are short on warriors and using paladins or bear druids as back-ups you might even get the job of roaving tank in Luci which means you back up everyone, and trust me with Mind Controls you will very likely end up tanking every mob in that fight for a time, though you are less likely to see Luci. I've been in the position where I tanked Luci and both guards for a period of time in that fight thanks to mind controls. It's fun trying to get enough aggro to keep the mob off the raid but keep it low enough so that the other tank can get it back when they get back to you.
You'll end up tanking Lave Packs and Garr adds as well.
None of the above : This can actually be a lot of fun, though now that we've gotten better you are less likely to have to jump in and main tank or pick up a patrol that got to close. But your job is to do DPS. Thunderclap will still help out here, again 10% slower attack means less damage to the MT and more chances to see holy fire or starfire from the healers and those are pretty spells you want to see them. :) But you get to pretend to be a rogue. Check Quarks post about the rogues, this is you but you have plate mail and the chance that if someone dies you might the one holding aggro and getting all the heals. You will be a little higher on the heal list than a rogue but not much bring bandages with you as well.
Skills : One other thing a whelp group tank should do is dem shout and piercing howl if you have it. This is true anytime you are supporting AoE though. Thunderclaps are nice too but a little trickier to get out. But this is true for anytime you are a tank supporting AoE. You want to have as little damage as possible hitting that mage or warlock. So do try to tank as many of the whelps as you can. It will take them some time to kill you even with a large pack on you and little healing coming your way.
Shield wall and Last Stand both have uses as main or secondary tank. Don't be afraid to use them. If you aren't directly tanking shield walling when you take aggro of if you have to step up when the MT goes down can often give the healers enough time to find you and send heals your way.
As Darian said, being a tank in avarice raids is a blast. I love all the roles I've played and I've played them all. Charging in with a high attack power set and a big two hander or dual wield to dish out as much damage as I can to main tanking Onyxia, Luci, Shazzrah, Garr, or whoever. We pride ourselves on having a fun close knit group of tanks that know that anyone there can step up and do any job that we ask of them. But if you bitch and piss and moan or decide that you are a better tank so do all you can to take aggro, you'll not get our respect and you'll not have as much fun.
Having been one of the few warriors that has played all the roles that Darian mentioned more than once in both MC and Onyxia I figure I can give a few tips. I'll start on the other end of his list. This is for MC/Ony as well.
Gear: I've taken the other route and use wardrobe to specialize my outfits for various encounters. Fire lords do pure fire damage so I pump that on them. I have high arcane outfits for Shazz, I have a mix of fire resist and def for ancient core hounds. I have a higher DPS set up. I have a DPS with a chance that I'll have to offtank set-up. You don't have to do this and keep in mind if you do you need to be quick with the changes since you can't change in combat. The other thing to keep in mind though is that stamina can almost never do you wrong. Sure optimal may be more def or more FR but when in doubt stamina with decent armor can't hurt you.
Main Tank : Darian covered it. You will have rage flowing in at a huge rate. So you can actually throw out shield slams and heroic strikes as well. These are high aggro skills and with as much rage as you have you don't need to worry about the most aggro/rage (which is actually revenge followed by shield bash). The job is all about aggro.
Main Assist : What Darian said again. When you learn the tanks better you will learn how soon you can call DPS but then again calling at 3 sunders vs 5 saves the raid all of about 10 seconds so the 5 sunder rule is generally pretty safe. As mentioned in places like Onyxia there will be someone who is calling DPS but the raid isn't really assisting them. As mentioned the MA won't always be a warrior either but we like to use them simply because of the CTRA tank windows it makes it easier. We do expect all warriors to understand when and how to call DPS in Molten Core though so pay attention or ask the other warriors when they are doing it.
Secondary Tanks : One skill that you should use if you aren't being beat on is thunderclap. 10% slower attacks on the mob is a good thing.
Some more specifics. We use 2 tanks per Molten Giants to keep them in place if they punt the other tank. The secondary tanks job is to stay just behind the primary tanks aggro on that mob. If you are backing up the main tank generally this means you sit in battle stance, make sure a thunder clap is on the mob and whack it. If the mob punts the MT flop into defensive stance to lower the damage of the one or two hits you'll take if you want. If the MT dies be ready to play tank. Since this is the second target to die you'll have plenty of time to build secondary aggro there is no need to try and take aggro from the other tank the healers aren't looking at your health as closely. If you are backing up the tank on the first target to die you are doing basically the same thing. After they land 5 sunders and DPS is called you may want to throw a sunder or higher aggro hit of your own in there now and then since the raid damage is coming and your job is to make sure that thing hits you when it punts the other tank.
If we have the tanks we'll also assign you as a back up in fights like Lucifron and Gehennas on the guards. In the Luci fight especially with the mind control you have to be on your toes. When we are short on warriors and using paladins or bear druids as back-ups you might even get the job of roaving tank in Luci which means you back up everyone, and trust me with Mind Controls you will very likely end up tanking every mob in that fight for a time, though you are less likely to see Luci. I've been in the position where I tanked Luci and both guards for a period of time in that fight thanks to mind controls. It's fun trying to get enough aggro to keep the mob off the raid but keep it low enough so that the other tank can get it back when they get back to you.
You'll end up tanking Lave Packs and Garr adds as well.
None of the above : This can actually be a lot of fun, though now that we've gotten better you are less likely to have to jump in and main tank or pick up a patrol that got to close. But your job is to do DPS. Thunderclap will still help out here, again 10% slower attack means less damage to the MT and more chances to see holy fire or starfire from the healers and those are pretty spells you want to see them. :) But you get to pretend to be a rogue. Check Quarks post about the rogues, this is you but you have plate mail and the chance that if someone dies you might the one holding aggro and getting all the heals. You will be a little higher on the heal list than a rogue but not much bring bandages with you as well.
Skills : One other thing a whelp group tank should do is dem shout and piercing howl if you have it. This is true anytime you are supporting AoE though. Thunderclaps are nice too but a little trickier to get out. But this is true for anytime you are a tank supporting AoE. You want to have as little damage as possible hitting that mage or warlock. So do try to tank as many of the whelps as you can. It will take them some time to kill you even with a large pack on you and little healing coming your way.
Shield wall and Last Stand both have uses as main or secondary tank. Don't be afraid to use them. If you aren't directly tanking shield walling when you take aggro of if you have to step up when the MT goes down can often give the healers enough time to find you and send heals your way.
As Darian said, being a tank in avarice raids is a blast. I love all the roles I've played and I've played them all. Charging in with a high attack power set and a big two hander or dual wield to dish out as much damage as I can to main tanking Onyxia, Luci, Shazzrah, Garr, or whoever. We pride ourselves on having a fun close knit group of tanks that know that anyone there can step up and do any job that we ask of them. But if you bitch and piss and moan or decide that you are a better tank so do all you can to take aggro, you'll not get our respect and you'll not have as much fun.
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It's all just zeroes and ones and duct tape in the end.
It's all just zeroes and ones and duct tape in the end.