Last night was a good lead-in for this, as I tanked Archimonde on our guild's second kill of him. The progression kill was done with our MT, and it was on a night I wasn't there. Yeah, I'm still bummed about that. After looking at it some more, we decided that I'd be a better candidate for it. I'll follow up with why shortly.
After two early aborted attempts (one where he freaked out and ran to the guy with water shield for some reason) we downed him. Even had an early death, but it didn't wipe us. The healing was great and people did a great job of staying alive. One of the comments I heard a few times was that I was easier to heal. I'm not so sure about that. Looking at the WWS from our first kill, our MT on that had actually higher avoided attacks than I did and took about the same damage per hit that I did. The main difference that I could see was that the fight was shorter and he took more spiky damage. I'm not sure why that is; armor should reduce all damage. I guess Archimonde is very variable in his output.
Why I might have been easier to heal may be because in his avoidance gear, the main tank has a lot less stamina and expertise than he's normally used to. For this fight I didn't do any real switching at all save a PvP fear breaking trinket. I only lost the stamina from Commendation of Kael'Thas and a food buff. I'll have to talk to him. My real belief is that I was easier to heal because people were used to the fight and knew what to do, so more people were in position to heal when it was needed.
(ETA: it's because he was chugging ironshield pots the entire time and had ancestral fortitude up. That explains a great deal. 7500 armor brings us about even. Still, it's interesting to note that with almost no change in gear the druid took about the same damage and close to the same amount of hits that the warrior did on this fight, while the warrior had to use consumables aplenty. Again, a good reason why a druid is better if you can deal with heals).
But why did we pick me to tank it? Well, let's go over what a druid is good for. A druid is great against physically hard-hitting bosses thanks to the high armor they stack. This is exceptionally true against bosses that cannot crush; a boss that cannot crush and hits mostly physically will do somewhere between 15 and 30% less damage to the druid compared to the warrior or paladin. Druids tend to be able to put out very good threat, especially at the T4-T5 levels, so for fights that require a lot of threat they can shine. They're excellent when damage is inconsistently applied thanks to their armor. They work well when it is impossible to avoid a crushing blow for whatever reason. They are exceptional offtanks, usually the best, due to how their threat mechanics work; they do a lot of threat (and gain a lot of rage) just from white attacks, and have easy ways of generating more rage. They tend to have more HP than any other tanking class at the same gear level, and get greater benefit from buffs due to their HP multipliers in bear form - this makes them good choices to soak hateful strikes. And they have stupid abilities that sometimes matter, like being able to shift from bear to bear and dump all rage. Phase change fights are often good as well thanks to dots from lacerate. Mobility fights are also often good because of being able to shift forms. And any fight where the tank will tank at some point and DPS at another a bear excels at.
What a bear cannot do is block. Any fight that requires this will be a fight a bear cannot do, at least not without some major support. (I've seen warriors intervene a bear tanking Illidan for the shears) Any fight with fear mechanics is usually not a fun one, but this isn't as big a deal as it used to be. Any fight where the boss primarily does spelldamage is usually a bad idea as well unless specifically geared against spelldamage. (While bears make great tanks for flames of azzinoth, I wouldn't recommend them doing so without 365 FR). Bears are okay at tanking multiple mobs, about as good as a warrior. They cannot interrupt reliably.
So what bosses in T5 & T6 are good candidates for a bear?
Morogrim: this is a boss that will cause crushing blows, swings erratically, and hits very, very hard. Morogrim is excellent for a druid tank because of the possibly slow swings, the rage starvation at times, the requirement that crushes will happen on warriors some times and the generally high avoidance of a Bear. While Morogrim can crush this is an example of a boss where he can in theory crush anyone, so a Bear is better.
FLK: The shaman is a good choice for bears due to his spiky, hard damage at times. The early threat lead a druid can put out is also good here so that he can be DPSed hard.
Leotheras: For the normal phase, the dots from lacerate help picking him up from the demon and after every whirlwind. Same for the demon as well, though the demon is not as good as a warrior or paladin because of the spelldamage component.
Void Reaver: Between higher avoidance, great threat and the ability to offtank well, druids tend to consistently do well on VR.
Kazrogal: Not that this is a hard boss, but druids do well here thanks to not caring about losing shield block on stuns.
Azgalor: doesn't crush, so this heavily favors a druid tank.
Archimonde: doesn't crush, but does fear. If you can make it so that the fears are not an issue by using priests and trinkets, a bear is great due to the high HP while having high innate avoidance. This was what we did last night, and it worked pretty well.
Supremus: Another hard hitting boss with phase transitions, combined with nice mobility. Works really well as a hurtful strike soaker due to the high HP and armor.
Gurtogg Bloodboil: offtanking quite a bit of a time helps a lot here.
Reliquary of Souls: Phase 1 is nothing special, though a high HP pool allows for multiple fixates. Phase 2 cannot be reasonably done by a druid without the raid overgearing the fight heavily. Phase 3 is interesting though; the druid has the ability to dump all their rage instantly, avoiding the soul scream damage somewhat, but you have to be on your toes and make sure you have a GCD available. A druid's threat scales better than a warrior's does on this as well.
Where a druid is not so good (or simply, cannot do it)
Kael'Thas: Kael requires a shield to block pyroblasts.
Illidan: Illidan requires 102.4% dodge from a druid. This is impossible, so unless you want to eat 60% hits regularly you can't do this. I've seen reports of warriors intervening on a druid for the shear, but this seems a bit crazy. Still, could be done - but it's not ideal.
Hydross: Hydross ignores armor and causes spelldamage. This means a druid will take more damage than either a paladin or a warrior, even in resist gear. And resist gear for a druid is very hard to find.
Illidari Council: Most of the Illdari council requires some kind of interrupting, so bears don't do as well here. The priest does best with a warrior due to interrupts, and the paladin does best with a warrior due to spell reflect. Veras is not so bad to tank with a druid however, as it helps to have snap aggro and mobility here.
And on Sunwell...Druids are a mixed bag of sorts. On the one hand, there's a lot of spelldamage; Felmyst, Kalecgos both do a ton, the twins do a ton, and Muru throws around a lot. On the other hand, Kalecgos, Brutallus and Felmyst all do not crush. Stacking avoidance for Brutallus and tanking is especially wise and the fight almost favors two feral druids as the most optimal choice, as feral druids do more DPS when tanking, they can do better DPS when not tanking, and they can deal with a dual-wielding, non-crushing boss better than any other tank.