I get a bit tired of seeing various ill-informed arguments about what tanks are good for what fights, and why they would or wouldn't be. I see it from people who know nothing about the class and from people who should know better. This is more of a rant than an in-depth math evaluation, but it's my damn blog.
Paladins are great at dealing with multiple mobs simultaneously. They're also excellent whenever you must remain uncrushable and you may take multiple hits in a short time. They excel against fast-hitting mobs. They excel on anything that has an aggro dump, as they have great snap threat abilities. Any fights with debuffs are usually good for a paladin as well. They work well against demons & undead. Anything that is sensitive to parry issues is also good for a paladin, as they do not have a particularly high chance of being parried due to not making a lot of melee attacks relative to a warrior or druid.
What they are not great at is dealing massive amounts of sustained high threat, at least compared to warriors or druids in their normal sets. (Paladins can do well if they know ahead of time they can go all-out on threat, but it requires consumable changes and some gear changes) They are not as good in high-mobility fights where a boss must be moved often thanks to consecrate. They are not great against caster mobs; they do less reactive threat and they take more damage. They are not good against mobs that fear. And they are horrible against mobs that silence.
Some bosses in T5 and T6 content that paladins excel at:
Hydross: great pickup on the phase change thanks to very fast aggro. They can also deal with the adds fairly well. Being uncrushable doesn't matter in this fight, and holy shield is useless, but avenger's shield is great.
Morogrim: Morogrim eats shield block charges up quickly, but a holy shield has a lot more charges than are likely to be eaten. Also, paladins tend to have more avoidance than a warrior does. Druids are still king here, mind you, but paladins are great. Of course, if you have a prot paladin here chances are you'll be tanking the murlocs.
Leotheras: Either the demon or normal form. Alliance paladins are especially good for this thanks to the SoV mechanics; having a DoT as your threat generator means phase changes are much easier. On the demon form a paladin gets extra threat (demon) plus easy ranged pickup and a nice phase transition, and combines this with innate fire protection and usually high HP.
Fathom-Lord Karathress: aside from cool videos of dealing with all the adds, dealing with the shaman is a good option given how quickly he can hit. Another is the hunter and his pet; using consecrate here makes this cake on both the pickup and the keeping aggro on both targets.
Void Reaver: VR's tough. If a paladin can get aggro, it's hard to lose it; consecrate helps a lot with knockbacks, a paladin can go all-out on threat boosts, and VR doesn't hit all that hard. If they can't get aggro though, it's often a lot of mana starvation. It's very much a feast or famine kind of fight.
Anetheron: Rage doesn't hit all that hard and has a lot of spelldamage, but Anetheron is more physical based. He's not a hard tanking fight at all, but having exorcism can be very nice here for added threat and damage. Tanking infernals is a good option too given the distance aggro a paladin can do.
Supremus: Ranged, quick aggro are great on this fight. Exorcism and Avenger's shield are excellent for reestablishing a position.
Gurtogg Bloodboil: being able to immediately remove all debuffs makes a paladin a great choice here; it makes the healers have a much easier time of it. Having two ways to get out of trouble in case the stacks get too high is another benefit. He also hits quickly, so more reactive damage is likely to happen.
Reliquary of Souls:Phase 1 is a wash. Phase 2 is almost certainly best with a warrior. Phase 3 is where a paladin can really shine. While a druid can dump all their rage most of the time (assuming decent GCD management) a paladin never has to worry about this and has the added benefit of having their actual threat scale almost exactly with the DPS's threat. And a paladin takes a lot less damage from the mana drain than the rage drain.
Illidan:Illidan's a demon, attacks quickly with a dual wield, and requires uncrushability (actually unshearability, which is slightly different). And there are phase changes galore. Paladins are great for this, though it's debatable whether they're better than warriors.
And honestly, I don't know enough about the fights in Sunwell to say intelligently what a paladin is best at there. I've seen reports that a paladin is very good at dealing with Brutallus (can't crush, but hits fast) and of course one is necessary for dealing with Mu'ru and with Felmyst. I understand that Kalecgos works well for prot paladins for the same reason that Gurtogg does; a prot paladin can cleanse himself of the debuffs via DS and tank him in the normal world for basically the entire time.
As to what bosses Paladins shouldn't tank:
Lurker: Consecrate tends to not hit lurker, so threat is low.
Vashj: the taunt mechanics often fail because of how they work (having to target the friendly usually means that you're out of range), so Phase 2 is often tough. P1 and P3 are great thanks to Blessing of Freedom however.
Al'ar: a lot of spelldamage is bad news here for tanking Al'ar directly.
Kazrogal: mana drain makes this a lot harder for a paladin. It's also a threat race, so more threat is really necessary here.
Azgalor: frequent silences make this a lot harder for a paladin.
Archimonde: frequent fears and lack of oh-shit buttons make this really annoying. He also doesn't crush but hits very hard.
Mother Sharaz: frequent silences.
It's a real shame that so many bosses in Hyjal are designed around screwing over a prot paladin, given that a prot paladin is so instrumental to getting to those bosses.
But let's go over some misconceptions:
-a paladin is fine on Hydross. While they don't normally have as good single-target threat on a boss, Hydross has short transitions that wipe threat, so sustained threat is not such an issue. A paladin has much better frontloaded threat than a warrior does. Furthermore, a lot of a paladin's threat is from base abilities, so having resistance gear does not gimp their threat the way it can a warrior.
-a paladin is fine tanking Al'ar. They can taunt off other tanks just fine on melt armor. The reason you'd likely not see a paladin tanking Al'ar is because they're so good at dealing with the adds, but there's no mechanical reason they can't do it.
-Kael: a pally does lack last stand/shield wall for sucking up a second pyroblast. They do, however, have Divine Shield, and Kael won't change his target during the pyroblast casting. This is easily better than last stand/shield wall for dealing with emergencies.