My experience with Paladin tanks is limited at best...but here is a guide I wrote up for my guild. I'm not an awesome tank (I can break 1000 TPS but I go down to 0 rage pretty quick), but I have tanked all of Karazhan as both Warrior and Bear, as well as two out of the six bosses of ZA and some of the trash on SSC. Yes that's not very much and I'm pretty sure my method of tanking will change, but here goes either way...
Welcome. If you've walked into this topic, you're well aware of the Holy Trinity that has been passed down since the dawn of Everquest. The Trinity is as follows:
Tank (takes damage)
Damage Dealer (deals damage)
Healer (heals damage)
This discussion is on the part of the tank, who's sole purpose is to hold the monster's attention by doing songs and dances so that he doesn't notice that overzealous rogue sticking a sword in his ass.
Tanking Terms aka "Listen up, I ain't gonna repeat myself"
Critical Hit - 2x damage bonus attack. It takes -5.6% crit to become uncrittable to mobs.
Defense Rating - Increases Defense skill. It takes 30 Defense Rating to equate -1% crit and +.25% to miss/dodge/parry/block.
Resillience Rating - Increases Resillience. It takes 39.4 Resillience Rating to equate -2% crit, -4% crit damage, and -2% DoT damage.
Dodge - A form of mitigation. It takes 18.9 Dodge Rating to equate 1% dodge. A dodge is a 100% mitigation (negates all damage), yet administers no rage. Warriors receive 1% dodge from 30 agility, Paladins receive 1% dodge from 25 agility, and Druids receive 1% dodge from 14.7 Agility.
Parry - A form of mitigation. You gain 1% chance to parry for every 31.5 Parry rating. Parry is 100% mitigation and reduces your swing timer by 40% (meaning you get 40% haste for your next swing). The haste effect stacks twice. Druids cannot Parry.
Block - A form of mitigation. You gain 1% chance to block every 7.88 Block Rating. Blocking is minor mitigation. It will not block full damage unless you have enough Shield Value/Strength to completely negate all the damage. Block mitigation comes in after all other forms of mitigation. Druids cannot block.
Crushing Blow - A form of attack only mobs can do. Crushing blows deal roughly 1.5x normal damage. A tank's ultimate goal (excluding Druids who cannot reach it), is to become uncrushable. You need 102% mitigation (Parry/Dodge/Block/Miss) to hit uncrushable
Attack Table - Generally, a table the game rolls a dice for. On this table is: Dodge, Parry, Block, Full Hit, Crushing Blow, Critical Strike, Miss. By increasing some values, you can push other values off the table and thus making them moot.
Uncrushable - When a tank achieves 102% Mitigation (via Block, Dodge, Parry, Miss), the tank is unable to be hit by a crushing blow, and in fact is also immune to normal hits. All attacks will either do 0 damage or be reduced by blocking. This is the ultimate goal of a tank.
Tanking item choices aka "Heal the bear"
Each tank needs to gear differently and no one stat should be sacrificed for another. All stats play a key roll in a tank's life.
Warriors: Defense - Stamina - Mitigation > Agility - Strength > Armor > Everything else.
Many people shrug defense after hitting -5% crit, but doing so is actually a bad idea. More defense = more mitigation. If HP was all that mattered, Warlocks would be tanking. Stamina is obviously good, more HP = live longer. Mitigation is just as it sounds and is defintely worth getting. Agility helps dodge and crit, while Strength increases AP and Block value.
Paladin: Defense - Stamina - +Spell Damage > Int - Mitigation - Agility - MP/5 > Strength > Everything else
Yeesh. Paladin tanks have it rough. Not only do you need everything a warrior does, but you also need Int, +Spell damage, and +mp/5, all of which are really scarce on gear. MP/5 isn't as critical with Spirtiual Attunement, but it does help. It should always be remembered that Paladin's generate hate PURELY through their spell damage (unless using Seal of Blood). I'm not going to lie. If you're going Prot Pally, you really need to do gear homework.
Druid: Resillience/Defense (till uncrittable) - Stamina - Armor - Agility > Strength > (Resillience/Defense after uncrittable) and everything else
Druid tanks have itemization fairly easy. Two of the four key stats are available on most rogue gear. Armor is tricky to come by, especially with Blizzard's lack of making armor kits/enchants stack into bear form. Agility gives an insane amount of dodge for druids and ups crit rating as well. A bear only needs 60 Defense Rating or 68ish Resillience (with 3/3 Survival of the Fittest) to hit uncrittable.
Tanking Tools aka "Mash the go button."
Warriors
Heroic Strike - I misjudged Heroic Strike. It's a rage dump, lowering your rage, but it generates a decent amount of hate and alternating Heroic and Devestate/Sunder can cause your threat to skyrocket. Sadly, you really need to be taking heavy damage to keep up the spam of both.
Mocking Blow - A tanking ability in Battle Stance. A crappy tanking ability in a non-tanking stance. You will probably never use this ability and that is a good thing. The hate it generates is limited, it pulls you out of the safety of defensive stance, has a two minute cooldown, and the "forces monster to target only you" effect doesn't work some of the time. Perhaps the most fun use for it...is in PvP...nothing annoys a warlock/hunter more than seeing his pet leave the priest to go on to a warrior.
Sunder - Ahh good ole Sunder. Sunder generates a decent amount of hate, lowers armor to allow more damage which makes more threat, and has enough talents to squeeze it under 10 rage. If you're not protection specced, this will be your key tool. Drag it to the bar and erect a shrine to it.
Shield Bash - Dazes the target at later levels (causing more Heroic damage) and stops spell casting. Many people work it into rotation. If your target can be shield bashed, save it for stopping the spell (some rogues still don't kick). If it can't be, go crazy.
Thunder Clap - A once weak, but now very strong tanking tool. It adds a minor hate spike, lowers enemy attack speed, and is a special kind of nature damage (effects those immune to nature but can still be resisted). The only hampering is its hefty rage cost. A great tool for AoE tanking and a solid tool for bosses.
Demoralizing Shout - An overlooked ability when it shouldn't. 10 rage to lower enemy AP...doesn't seem too helpful does it? An untalented Demo shout lowers Gruul's Hateful strike damage by 1000 damage. It generates pitiful hate, but the lowered damage is worth the minor cost. Usually the second thing you should drop after a sunder effect is on the enemy.
Shield Block - Spamming this ability raises your uncrushability to max or close to max for a bit. Illidan's fight actually causes a wipe if you spam Shield Block, so do use it responsibly. It increases your block chance by 75% for 6 seconds or 1 block (2 if talented) and has a 6 second cooldown.
Revenge - Heroic Strike Version 2.0. Becoming available after you preform a mitigation act except being missed, it delivers a bit of damage with a threat bonus. Originally, the damage was pitiful. Blizzard has since jacked the damage up to a nice respectable amount and yet still kept it having a fairly high threat modifier. With talents, the cost is brought to 2 rage. Definitely an awesome choice and should be used any time the little button lights up.
Shield Wall - The "Oh sh*t" button. Reduces all damage taken for a time by 75%. If you're hitting this, it's go time. 30 minute cooldown means it's mostly a one shot deal. Your last ace in the hole. Use it wisely.
Taunt - Ahh taunt. Instantly sets your hate to the highest target. Does ABSOLUTELY NOTHING if you're the primary hate target. Has a 10 second cooldown and a lot of mobs are immune to it. My advice for taunt is: Unless you're picking up a CCed mob or it's a special case (Bear boss in ZA), you should never need to taunt. It's important to note for Damage Dealers, if you get aggro and you see the little taunt icon to SLOW YOUR DPS DOWN. If you crit right after taunt, you have to wait about 7 more seconds for it to come back up, during which the mob will enjoy your tight ass for its raping pleasure.
Intimidating Shout - Causes fear in enemies around you for 8 seconds. If they're feared, they're not hitting you. Also the main target will be stuck in one place cowering. Problems are that feared mobs run everywhere and that damage breaks the effect really easily.
Challenging Shout - Causes all enemies effected to have their threat towards you maximized. Great for dragging an entire group off the AoEing classes.
Last Stand (Talented) - Increasesyour HP by 30% for a bit. An "oh sh*t" button, but usable a bit more due to lower cooldown.
Concussive Blow (Talent) - Stuns the enemy. 45 second cooldown and only good for trash. Still if Shield Bash is on cooldown, it makes for a handy second spell stop.
Shield Slam (Talent) - Deals damage, dispels magic, and causes high threat. Only thing that sucks is the cooldown. It's damage is modified by your block value, which you're semi-stacking up anyway so expect to see very good numbers out of it. Your number 1 source of threat outside of Sunder.
Devestate (Talent) - Originally, a crappy talent. Devestate did horrible damage and worst of all, horrible threat. However, Blizzard changed this last patch. Devestate is now effected by Sunder talents (Improved Sunder mostly) and now does its own threat + one application of Sunder. Devestate is now the second best Rage to threat generating ability (just behind Shield Slam). It is highly advised to grab Devestate and replace your sunder button with it.
Commanding Shout - Increases HP of those around you. HP = good.
Intervene - A weird charge that's incredibly annoying to use. I wish Blizzard would fix it already or make it less of a pain to work right...
Spell Reflect - An extremely fun talent. It's usable on a lot of things, but not everything. First of all, only single target spells (few exceptions) can be reflected. Not all spells can be reflected though so keep your eyes open. Still, nothing feels better than blasting someone with their own spell.
Paladins
Seal/Judgement of Crusader - Increase attack speed and AP. The seal itself is garbage, but it's not the seal you want...it's the judgement. Seeing as how a paladin's full threat is holy damage, this judgement makes your hate go up. There are other judgements that work as well, but if you can, you want Crusader up.
Seal/Judgement of Righteousness - Deals holy damage. Plain and simple. Holy damage = threat.
Seal/Judgement of Wisdom - Only use when badly OOM. Otherwise Crusader all the way.
Exorcism - Holy ranged Damage to an Undead/Demon target. Useful for pulling if your target matches the criteria. Also a good burst of threat.
Seal/Judgement of Vengance/Blood - Blood is more of a PvP/Retadin spell, but being healed restores mana. Vengance is a DoT that stacks. Blood gets power from AP, Vengance from +spell damage. I'm not 100% sure how they compare to Seal of Righteousness, but I think its a mixed fight.
Concencrate - Monster apparently don't like their shoes getting scuffed with holy energies. Generates a good amount of threat all over the place. Only thing that sucks is the mana cost.
Righteous Defense - A ranged taunt that effects three targets instead of one. Defintely a very nice ability.
Righteous Fury - It's a magic effect, make sure to have it up...especially if it is dispelled. Talented, it turns into Defiance + Defensive stance. If you're tanking, this is up...all the time.
Retribution Aura - Deals holy damage when hit. I think it's safe to say everyone knows what Holy Damage is to a paladin tank at this point. DOES NOT STACK WITH DARKMOON: VENGANCE.
Blessing of Sanctuary (talent) - Lowers damage taken, deals damage when blocking...deals HOLY damage when blocking. Definitely the tanking blessing.
Avenging Wrath - Increases damage by 30%. La la la...the Paladin + damage equation again.
Holy Shield (talent) - An awesome ability, pushing a paladin to near uncrushable for a fairly extended period of time. Also causes holy damage when you block, while increasing your chance to block.
Avenger's Shield (talent) - Captain America go! Causes good amount of threat and is really funny to watch.
Druid
Maul - Bear version of Heroic Strike. Bears and their high STR and AP weapons make this ability a great one for getting hate, plus because a bear's attack speed is a set 2.5, makes it so you don't have to scramble to mash it.
Swipe - Bear AoE...used to be absolutely nasty and allowed AoE grinding. Sadly, it is no longer the case and barely holds hate. Make sure everyone focuses on the target you're actively attacking and not a side swiped one.
Bash - One minute cooldown stun. The only bear interupt besides Feral Charge.
Feral Charge - Bear intercept. Also interupts spells and immobilizes. Definitely worth the talent point investment...even if you do look goofy while charging.
Demoralizing Roar - See Demo Shout.
Mangle (talent) - Ahh mangle, truly a gem. High damage, increases bleed damage for Lacerate, and has a pretty nifty animation. When you see the button twinkle, mash it. This is one of the key go buttons.
Lacerate - Go button 2. Lacerate is actually a rage dump. It causes bleed damage on hit (special kind of bleed damage) and causes a stacking debuff, assuming the target can bleed.
Faerie Fire - Bear's crappy sunder. Doesn't stack, low armor reduction, 6 second cooldown. It's free though...besides one talent point. Just slap it on whenever it is up. It does cause some threat and is ranged, but don't expect to hold hate on FF alone.
Thorns - Damage = Threat. Nuff said.
Growl - See Taunt
Enrage - Causes your armor to drop (base armor, which doesn't effect that much) and generate 20 rage over 10 seconds. Good for getting rage if you need it, twice over. You take a bit more damage, but that gets your more rage.
Frenzied Rejuvination - Not a bad skill but definitely no longer a good one. Converts 10 rage per second into health, each point converted into 10/15/20/25 health. A 2500 heal over 10 seconds at 70. Really pathetic but it does generate threat and can help take some small stress off the healers.
This concludes the tanking guide as of now...I'll probably add in talents later, but you get the general gist of how it works. Also, if you have any advice, changes you'd like to see made, please suggest them. As I said, my experiences are limited.
Edit: Adjusted thanks to manorath.swe (Removed Light, Adjusted Wisdom, added Exorcism, added final values for -crit, uncrushable numbers)