Original Discussion

WARNING! This is the complicated version.

I've done some digging and found a really useful chart about threat values. It's not the same chart I once used but it's very similiar:
http://www.wowwiki.com/Template:Aggro:IncrementModifierTable

The chart helped me to establish my tanking regime with Venka. Which is:

Pull
Taunt
Sunder
Shield Slam
Sunder
Revenge
Sunder
Shield Slam
Sunder
Sunder
Revenge

Then I begin to rotate my abilities differently, I use Revenge and Shield slam as often as they are available and Devastate when they are on cooldown and I already have 5 sunders up. Occasionally I'll do a sunder instead of devastate if the cooldown is ticking away (ie I got feared or mind controlled or something where I'm not hitting the boss for a short period and the sunders are expiring).

I only use Demoralizing shout and Thunderclap when there are multiple mobs involved for me to be tanking. Shield bash I use only on mobs that I HAVE to interrupt and Shield Reflect I only use if I really need to give the healer a break and mitigate some more damage. I'll also use Shield Reflect if the mob does a particularly hurtful spell that I would love to turn back on them. (ie mage rift keepers in BM, reflecting their pyroblast back at them is sweet) Shield Block I only activate if I'm taking a lot of damage and I don't need the rage for anything else anyway.

Hope that helps any of our tanks out there.

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Taking it a step further...I won't do the math but I will point out the process if anyone is interested in this sort of thing. I know I found it fascinating even though I don't like math.

Tanking with Venka:

Pull
Hit the mob with a dagger dealing 80 damage. My threat just for doing damage is 80*2. But because I have the defiance talent (+15% threat for dmg done in defensive stance) and I'm in defensive stance (+30% to all threat) my threat is a good solid amount. This is why I try to pull the secondary target first, not the primary. It gives me that extra couple seconds to establish aggro on all targets.

Taunt
This ability does not give me any threat. It only forces the target to attack me while I establish some threat with other abilities. When someone pulls aggro, they are at 100% on the mob's threat meter. I can taunt and get to 110%(therfore getting aggro) but only for the duration of the taunt. If I don't get back above the person in threat then I will loose aggro again when the taunt is finished. This is why you stop DPS on a target for a little bit if you pull aggro so the tank can reestablish it.

Sunder
This ability does not cause damage but it does have a base value of 301. I will also get the +30% threat when I'm in defensive stance. This ability stacks 5 times and is very effective in maintaining aggro as long as you keep reapplying it. Just because it stacks 5 times doesn't mean you won't get additional threat for putting a 'sixth' sunder on. You DO get the additional threat for re-applying sunders. Your DPS and healers aren't going to stop healing or DPSing, you shouldn't stop applying your best aggro holding abilities.

Shield Slam
This ability has a base threat value of 301 just like a sunder. But unlike a sunder, it aslo does damage and is capable of crits. This ability also has a short cooldown so it can and should be used fairly often. 301 base threat + (damage done*2(+15% defiance))+ 30% defensive stance = yummy threat!

Revenge
IMO the best ability in your arsenal for holding onto threat ESPECIALLY if you have improved revenge talents. After you've been struck in combat you have a chance of getting the ability to use Revenge so it should be used as often as it becomes active on your spellbar. It gives 417 base threat which is over a hundred more than a sunder. PLUS it does damage PLUS (if you have imp rev) it has a chance to stun the target causing more threat(+25 more). So 417 base threat + 25 (if stun occurs) + (damage done*2(+15% Defiance)) + 30% defensive stance threat = Houston we have aggro.

Devastate
This can be a confusing ability as far as threat goes so let's keep to the basics. Devastate has a base threat value of 101. It does damage that is based on half your weapon damage + 35 damage for every sunder you have stacked. This does not give additional BASE threat for every sunder stacked but the +damage per sunder does have normal base threat attatched to it. To make this an effective aggro holding ability, stacking sunders at least 3 times, preferably 5 will give you the best results. So assuming you have 5 sunders stacked...101 + (damage*2+(15% defiance)) + 30% defensive stance = just as good as another sunder except you do some damage too and maybe stay above the pet in DPS.

Thunderclap
Base threat is damage done *1.75. This is, again, also affected by Defiance and Defensive Stance. It can also be made more effective by Improved Thunderclap which makes it do more damage, therefore, more threat. I use this ability when there is no worry about breaking CC and when I have multiple targets to tank. It gives me that extra couple seconds I need to slap sunders on the first target then quickly put a sunder or two on each of my other targets before getting back to the first before DPS or healing pulls aggro. The more mobs there are to tank, the easier it is to loose aggro. This is a very effective tool to battle that.

Demoralizing Shout
This ability only gives a base threat of 45 + 30% defensive stance. It will not hold mobs for long but works well to pick up multiple targets when you can't use Thunderclap. Also works well WITH thunderclap.

Other abilities that the warrior has to generate threat I usually don't use very often and I'll go over my reasons below. It doesn't mean you can't use them in your own tanking style, it just means I don't use them and here's why.

Battle Shout
Yes, don't gasp, it causes threat. It is +60 threat per buffed member of the party. I do not use it until the end of the fight because I usually cannot spare the rage. If I can spare the rage needed, then I will use it. I do try to maintain it (stupid 2 min buff) to help the dps but it's not exactly on my priority list.

Heroic Strike
Some people use this instead of Devastate and just refresh their sunders with another sunder as needed. This is fine except the rage cost and time it takes to go off makes me definately prefer devastate.

Cleave
Really slow, lots of rage. I'm too impatient to use it but it could be effective against tanking mulitple targets. 130 base threat + it does damage with all the multipliers involved.

All done, I'll happily discuss stuff with anyone interested in learning more about aggro. And you thought tanking was hard...

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Hrmm, I guess I better post something constructive, so here is how I pull/gain opening agro on Cyndeace.

ON TRASH:

1) pull with Avengers Shield-> It generates 1k threat per mob(1-3) on a noncrit
2) consecrate just before mobs get into range
3) judge seal of righteousness
4) keep seal of righteousness up
5)just before (about 5-10%health) the mob is on I judge righteousness on the next mob, for guaranteed agro, rinse and repeat
5) usually, as long as people are attacking my target, all is good, it's only if people unload on a target I am not hitting that they pull agro

ON A BOSS:

1)Avenging Wrath: 30% more holy damage for 30 secs
2)Pull with Avengers Shield-standard 1k threat+bonus from AW
3)consecrate- tell group members to wait 5 secs
4)Judge righteousness
5)Pop Holy Shield-30% increased block+190 holy damage per block, 8 charges or 8 secs
6)after Boss is at 80%ish judge seal of vengeance=700-800 holy damage per tick
7)judge SoV and consecrate as needed
8)Holy Shield as lmuch as possible
9)Profit!!!

Pally tanking is a bit different than warrior/druid tanking, but the same rules apply, stay under his/her threat. That's why you will usually see me BoSalvation all dpsers, aside from hunters I know and trust lol. It lowers your threat by 30%, allowing you to do a good 20%-30% more dps without pulling agro.

One thing I have found being a pally tank. On every pull, if the dps waits 2-3 secs and then assists my target, there is almost no chance you will pull agro. The only times you will is if you get a string of crits, I am half asleep, or you really outgear me.

And the last, but most important: If you pull agro from me, RUN TO ME, not away from me. I have a ranged taunt, but alot of my threat on adds and the mob I am attacking comes from being hit and consecrate. If I have to run after a mob, the others get out of that and it makes it harder for me to regain agro.

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Good posts guys! I'd like to add my 2 cents for some dps and also for some tanking, especially if you're in Kara. And believe me, tanking in Kara and tanking in a 5 man are very different.

Biggest difference between Kara and 5 Mans is that you normally are tanking 1 (2 at most) on any given pull in Kara and they do a a TON of damage so generating rage is usually not an issue at all. Generally I follow the same rotation Rill does for tanking in Kara, I use shield slam and revenge as much as possible. In addition though, to help mitigate damage and to make sure I can use Revenge I try to use shield block whenever I can, that and when I get to the point when I'm rotating between shield slam, revenge and devestate (normally only on boss fights) I throw in Heroic Strikes about every other attack, because at that point I'm not hurting for rage. Also try and keep Battle Shout up! It causes your normal attacks to do more damage (more threat) and the more damage you do the more damage you do the more rage you generate as well.

FOR DPS! Especially ranged!!!

Here's how threat works:
At start of the fight, first person that has aggro is at 100% on the threat list
For MELEE dps to pull aggro off the MT, they have to do 110% of the tank's threat
For RANGED dps to pull aggro off the MT, they have to do 130% of the tanks's threat (goes for healers out of melee range as well)

So as you can see, its easier for melee dps to pull aggro, but also easier for the tank to get it back. Its also much harder for ranged dps to pull aggro, but its makes it that much harder for the tank to pick it up if the dps pulls the mob. So ranged, you have a bit more of a buffer but it also means if you pull aggro (especially in Kara where a lot of mobs are immune to taunts) you are dead and there's not a thing the tank can do about it.

Its a lot of info, but I still say the best way to learn to tank is to just do it. Practice and experience is the best way to become a better tank. AND IF SOMEONE GIVES YOU ADVICE LISTEN! And don't get bent out of shape. If you're in a PUG you can choose to listen or not, but if you are in a group with any of the experienced players from our guild, do listen. For example, Jrod does not have a tank, but he's heard Lem and I rant, complain and talk about tanking so much that I'm sure he could answer some of your tanking questions.

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I wanted to bump this back into life.
I like Cryan's suggestion of putting this together in a sticky post, we'll look into doing that.

I also wanted to add a few things about druid tanking.

Druid tanking is similar to warriors.
Druid tanking revolves around mangles and lascerates to maintain and generate aggro.
A common misconception is that Growl builds aggro. It does not. Growl simply forces a target to attack you for a few seconds. This can be used to initially get aggro, or regain aggro if someone has pulled form you.

I typically start the fight with a growl to start getting aggro. However in Karazhan and other places where mobs are immune to growl a mangle works well.

Over a long boss fight I constantly spam lascerate, mangle, fearie fire, demoralizing growl, swipe (if no cc nearby). Mangle and lascerate must be used as often as possible to maintain aggro throughout a long fight.

With any boss fight, or long mob fight you should always have 5 lascerates stacked.

Also, try to keep thorns up on yourself. It might seem silly but every little bit adds up.

I am noticing a trend in long fights where aggro levels out, this makes using mangle especially critical. Mangle generates a lot of damage and threat. Many times I've watched Omen and seen people approaching my level of threat even though I'm spamming laserates. Rotating different abilities then comes in key. Mangle, FF, demoralizing roar, swipe, etc. It takes a lot of rage, but it works.

Anyway, thats my cents on druid bear tanks.
Just remember, Mangle and Lascerate are critical.