In the first phase, you've got a large and very nasty Magnataur named Gormok the Impaler. As his name implies, he will be throwing down a debuff called Impale which will stack up to ten times if you let him keep beating on you. This means that you'll be taking 1750 to 2250 damage every two seconds per application in the 10-man version or 2188 to 2812 in 25-man. Being that it will last 30 or 40 seconds depending on your raid size and it gets refreshed with each application, you will have to use two tanks in order to stay alive. As a tank, this will feel a lot like the Gluth fight from Naxxramas.
After accumulating anywhere from 2-6 stacks depending on both your gear and your healer's skill, you'll swap tanks to let the debuff dissipate. As a paladin, you can use some tricks to help make this easier. Hitting Divine Shield on yourself will instantly clear the debuff once the other tank has taken over. You should make sure to clear your bubble immediately though. A good macro for doing this is:
/cancelaura Divine Shield
/cast Divine Shield
Clicking this macro once will cast Divine Shield and clicking it again will then cancel your Divine Shield. This is a great macro for tanking as it can occasionally save your shiny, armored butt if you accidentally stand in Bad Stuff (tm) or just need to clear debuffs (other uses include: Hodir's Flash Freeze and Razorscale's Fuse Armor before you've completely become a statue).
Now that you have your debuff cleared and the other tank has taken their appropriately deep stack of Impale, it's time to taunt and start the process back over again. One thing you can do to help out the healers is after taunting, you can cast Hand of Protection on the other tank to clear their Impale stack. I'm sure the healers will love you for it. Just make sure you let the other tank know you're doing this in case you die for some unexplained reason and they can't figure out why the boss is ignoring their threat.
After a couple swaps back and forth and hearing people in Vent complaining about Snobolds and patches of fire (neither of which should really affect you tanking), Gormok will die and your next two challenges will appear.
The second phase has Acidmaw and Dreadscale as your opponents. They're a pair of highly agitated Jormungars (aka: worms) that the Argent Crusade dug up from somewhere. Each worm must be tanked separately, so each tank should designate their target. At the start, Dreadscale will be slithering across the surface while Acidmaw will be sticking its head up through the floor of the coliseum and will be stationary. Every so often, they'll both duck underground and change positions. There is a little cloud of dust on the surface that you can follow to see where they're going. After they pop back up, the opposite worm will then be mobile while the originally mobile one will be stationary.
Acidmaw uses nature based attacks and Dreadscale uses fire based attacks so make sure that you or another Paladin swaps over to Fire Resistance Aura. Both worms have a spew area effect when stationary and a spray cone effect they do mobile. Acidmaw's spray leaves a Paralytic Toxin debuff which reduces movement speed by 10% in addition to 3k nature damage every 2 seconds. The only way to remove Paralytic Toxin is to find someone who has been attacked by Dreadscale and gotten the Burning Bile debuff from his spray or bite. Burning Bile causes 3238 to 3762 fire damage every 2 seconds to all allies within 10 yards. This means that unless you're fixing someone who has Paralytic Toxin, you should keep your distance from your fellow raid members. As the Dreadscale tank should often be the only one with the Burning Bile debuff, they'll occasionally need to go over and help remove the Paralytic Toxin from the Acidmaw tank and melee dps.
The most important part of this fight is to keep both the worms facing away from the raid to prevent the debuffs above from hitting them. You can organize this ahead of time that you'll always have them face a certain direction (ie: north, east, etc) or you can play it by ear and call out the direction over chat.
When you're tanking the mobile worm, it's a bit like Grobbulus where you have to kite the boss around while keeping it facing away from the raid. Every so often, the worm will drop a Slime Pool on the ground which inflicts around 2k nature damage every second. Definitely not something you want to stand in while tanking.
Unless you're going for the Not One, But Two Jormungars achievement that requires killing the two within ten seconds of each other, the DPS should be concentrating primarily on Acidmaw during this fight and then swapping over to Dreadscale when Acidmaw is down. As soon as you have killed one worm, the other will enrage and start doing 50% more damage than he was earlier. If you still have some people affected by Paralytic Toxin at this point, you may want to swap tanks on Dreadscale and have the other tank unsticking people.
Once Acidmaw and Dreadscale are both worm food themselves, a giant Yeti will lumber into the arena.
As far as third phase goes, it can either be very easy or a spectacular amount of suckage depending on how well your raid group moves. Icehowl looks like a massive and extremely scary yeti, but he's a pushover if people just stay out of his way.
Feel free to tank Icehowl where ever you like, because he'll be moving soon anyways. During the tankable phase, the only major attack you need to watch out for is his Ferocious Butt attack that hits for between 60k-70k on 10-man and 70k-80k on 25-man as well as stunning you for a little under 3 seconds. All of that damage is before mitigation so don't panic too much as that comes down to around 22k or so. While one tank can do this fight, it is safer to have a second tank taunt when the headbutt hits and tank him while you're stunned. Switch back accordingly or as cooldowns permit. That said, as a Paladin with the Ardent Defender talent, you can take a substantial amount of damage during the short time you're stunned if your healers are on their game.
He'll also occasionally pick a raid member and do a cone attack called Arctic Breath that freezes everyone in it's path for five seconds as well as doing 3000 to 4000 damage per second. This is a pain, but you can get out of it with that little Divine Shield macro I mentioned earlier and back to tanking. As this is a channeled spell, if you do get caught in it you don't have to worry about Icehowl rampaging through the raid eating all healers and rogues in his wake.
The part that everyone should be looking for is Icehowl's Massive Crash followed by his Trample. This is where he knocks everyone in the raid back against the walls, stuns them for a moment, and then picks one raid member to charge. This includes a nice little emote message to let you know who he's going to pounce. After you get knocked back to the walls, you'll get a speed buff called Surge of Adrenaline for 8 seconds. Use that speed buff to get the heck out of Icehowl's path. This whole section is telegraphed by his movements and there is plenty of time for everyone to get clear as well as see which direction he is going to charge.
Let's just take a moment here to say what happens if you don't get clear of his charge. Everyone in his path will be hit for 50k damage. Besides that, he goes into a Frothing Rage that increases both physical damage and attack speed by 50% unless a Hunter with Tranquilizing Shot or Rogue with Anesthetic Poison can bring him out of it. This sucks. This is a blow all cooldowns type of tanking moment presuming you weren't the person hit and are now lying dead on the floor (or wall, or ceiling, or where ever you corpse landed). If the person hit was a tank or a healer this may cause a wipe.
Luckily, it's fairly easy to avoid this. In that case, Icehowl runs into the wall and falls into a Staggered Daze during which he's stunned and takes 100% extra damage for fifteen seconds. There is some debate whether damage taken during this time period causes threat or not. Regardless, this is a perfect moment to pop Avenging Wrath and try for some of that non-existent Protection Paladin DPS.
When he wakes back up, he usually seems to do an Arctic Breath and then you tank a while until the process repeats. As long as your raid avoids the tramples and you can survive the headbutts, the fight goes quickly and easily. All that is left is loot at that point.
What types of tank gear can you expect to find in his fuzzy remains besides three Emblems of Triumph? Well, as gear is now segregated by faction even though it has the same stats, we end up with the following.
In 10 man you might find...
In 25 man you could get...
The Light and How to Swing It tries to help Paladins cope with the dark times coming in Cataclysm. See the upcoming Paladin changes the expansion will bring.