First, I'd like to have some tanking reference for other tanks in regards to these fights, as well as for my own personal log. Knowing how I handled the normal-mode version of an encounter may help in formulating hard-mode and future encounter solutions.
Second, I'd like to demonstrate just how well Blizzard has stepped up the tanking experience. The days of "stand and tank" or "tank and swap" have been relegated to trash pulls at best; the boss encounters dive into some exciting new mechanics.
Now, an initial sampling: I'll touch on Razorscale, Iron Council, Freya, Hodir, and Thorim below. Mimiron, Vezax, and Yogg I have yet to properly engage, so I don't feel I have adequate experience with them yet; expect more on these soon!
Razorscale: Everyone loves to call AoE tanking irrelevant for a true MT. The new Unrelenting Assault build for Warriors sacrifices nearly all the available Warrior AoE talents, and this encounter is one of the primary reasons I believe that such a spec will fail. From a tank's perspective, this encounter is solely concerned with controlling a horde of dwarves. This task is enormously difficult to handle at first, especially when dealing with multiple tanks trying to share a bunch of adds all spawning on top of each other: one tank invariably picks up most of the adds, and this is a dangerous state (so much incoming melee + an unlucky fireball = dead tank, not to mention the rage/mana starvation of the other tanks so they can't pick up if the one tank dies or loses a mob). At the same time, all of the gaps have to be covered to protect the healers from getting gibbed by anything other than bad boss RNG.
At the end of the day, this fight requires some immense AoE tanking across all of your tanks, and they must be comfortable with sharing the tanking burden. Tanking Razorscale himself requires some clever kiting mechanics while maintaining strong threat, but the difficulty here is far less - it's just an elaborate version of "stay out of the fire," with some tank-swapping coming up along the way.
Iron Council (Assembly of Iron, for the nitpickers): So much for Gag Order being a useless talent. After tanking this encounter on 10s and 25s, I'm extremely glad I ditched Improved Disarm for Gag Order in both of my specs. The reasoning for this? OTing Phase 2 of the easy-mode approach to this fight (Stormbreaker -> Runemaster -> Stormcaller Brundir).
Phase 1 OTing is simple enough, because Shield Bash will easily shut the mob up if he happens to be standing in a Rune of Power. Cycling stuns for the down-time means that good reactions leave Brundir entirely incapacitated for Phase 1.
When Phase 2 hits, I usually have a rogue assist interrupts because Lightning Whirl and Chain Lightning are not in the same spell school (can someone explain that for me?). This usually works fine. However, on two separate occasions we have had a Rune of Death spawn directly underneath us. The result? The rogue and I are forced to run for the walls, leaving Brundir 15 yards from us, happily spamming his spells of death on the entire raid. The solution? Serve up one (Gag) Order of Heroic Throw. The three-second silence easily drags Brundir out of the Rune of Death, however sad he may be, and interrupts can proceed as normal. I enjoyed this moment of success a bit too much, I suspect, but it was still pretty amazing in my opinion.
Freya: Again with the adds. I guess my job for this fight is considered OT, but the adds are just so much more exciting to deal with than Freya herself. The Conservator is admittedly boring, so long as DPS waits for me to get under a mushroom.
The Lashers are fun, as Shockwave will stun all of them in their tracks if they're correctly stacked. I don't know that the stun is long enough to let DPS kill them without people being in range, but at least the removal of some melee damage eases the healing by a bit. This portion of the fight really revolves around precision stun/nova timing to kill them without killing the raid, or else single-target sniping through the adds once they're weak.
Thereafter, bring on the trio of adds. I'm still working on a reasonable way to handle all three on 25's, but in the meanwhile a DK spamming CoI allows for good survival against the Snaplasher during this period. I, meanwhile, spend my time stunning the hell out of the Water Elemental to keep it from running away, and then Shield Bashing in the face of the Stormlasher's Storm Bolts. Never have I been so grateful for Cleave, slapping threat onto both of these targets indiscriminately as I swap back and forth between them.
Once the adds are cleaned up, all that remains for me is some Devastate-spam to make Freya more vulnerable, and then chaining my pitiful Berserker Stance DPS on her back.
Hodir: I thought tanking this fight on 10man was fun. Compared to tanking this fight on 25man, the 10man version is boring and dull - 25man Hodir is an absolute blast. Here I am, rolling my mitigation spec and frost resist gear to all but guarantee that this is my weakest set for threat generation. I didn't want to risk more threat gear or the threat spec, as straining healers in this encounter is a bad idea.
Meanwhile, every one of the DPS is sitting on huge amounts of DPS-boosting buffs. The result: Our top boomkin rocking 7.5k average DPS on Hodir's face, with most of the rest of the raid easily hitting 5k to 6k DPS.
Even with Vigilance up on a warlock and Intervene on constant cooldown, I was breaking my face against the keyboard to keep up. Concussion Blow, Shockwave, Shield Slam, Revenge, and even Heroic Throw were all on constant cooldown. Devastate was squeezed in wherever it could fit; I eventually ditched TC and DS from the rotation entirely - I couldn't afford to lose a single threat-oriented GCD. The boss's bursty damage made me curse as I was forced to give up a few Heroic Strikes to keep from missing another Shield Slam.
Admittedly, I gave the buffs no mind, and they may have helped my performance. Nevertheless, I never lost to the Boomkin in threat (I'm still kicking myself over losing a mage in the opening moments, but Taunt has frustrating mechanics when the boss isn't targeting anyone).
So why do I enjoy this encounter? What on Earth is appealing about breaking my fingers trying to maintain a superior threat rotation in mitigation gear while moving, jumping, and intervening as often as I can? Simply put, it's the challenge. If I miss a single GCD or side-step, I risk the life of a DPS or myself. There isn't another boss fight in the game where I've felt the same kind of accomplishment for knowing what I'm doing with my tank. I was given all of these tanking tools when my class was made, and this encounter gives me a real chance to put every one of them to use. No gimmicks made simple by an intervene (Overload), and no mechanics to make the tank's life remain easy when the DPS are boosted (Loatheb, Thaddius).
People said they had trouble holding threat on Malygos; I never had a problem with him, even with portalling Warlocks. Now there's Hodir, and good God does he give me a workout. Getting into Hard Mode on this one is going to necessitate my threat gear and spec, I fear; I apologize in advance to the hell I'll likely put healers through when we get there.
Thorim: Moar adds! Moar! The arena is a chaotic swirl of death, and the hallway is almost boring by comparison. If it weren't for the hellish pain that the Arena was going through, there'd be no reason to rush the hallway at all. This is thankfully a fairly front-loaded fight, unlike Malygos. If the arena can survive long enough for the hallway to beat down two minibosses, then Thorim himself is relatively simple: spread out to avoid spreading Chain Lightning, and stay out of the glowing blue sparks on the floor. Tanks can taunt back and forth easily enough for Unbalancing Strikes, and it's just a weak sort of DPS race to drop Thorim before he drops the tanks (or DPS with his Chain Lightning).
I find this to be yet another fight centered around controlling and decimating adds. Do I hear a theme of adds in raid boss encounters? I thought I was good at controlling adds packs - encounters like this teach me that I still have work to do.
Other: Ignis, Deconstructor, Kologarn, and Auriaya are all fun tank encounters, too, though dramatically simpler. The Keepers are really the pinnacle of tanking challenges (Mimiron will likely join this list when we're through with him). Vezax appears to be more of a simple tank fight, if you choose to rotate cooldowns on the tank; the stress here is on the healers. Thereafter, there's Yogg, Hard Modes, and Algalon.
Ulduar does indeed push forward some of the best content tanks have gotten to experience in a long, long time. I've said it before, and I'll say it again: I'm enjoying every minute that I spend tanking in Ulduar, from the wipes to the one-shots.