Since it was suggested as a shared topic from Blog Azeroth, I thought I should do a rough evaluation guide for the three tanking classes from the perspective of a tanking Paladin. To get the obvious class plug out the way, choose a paladin, doesn't matter if the boss silences fears or drains mana, choose us, we are shiny and have the light, and when the going gets rough, real tanks wear pink. On a more serious note though:
Warrior tanking is essentially the baseline tank for the game, there is no fight they cannot tank, nor is any fight particularly difficult for then, a Warrior provides a large amount of utility to the raid while tanking, using spell reflect, shouts and debuffs to maximum effect. Warrior tanks essentially rely on Sunder to provide a large proportion of their threat making a rogue's expose armour useless in a fight like this. Warriors have a fairly standard threat rotation with threat "spikes" every global cooldown leading to a fairly linear threat curve.
Warriors lack AoE tanking capabilities, but this is not to say they can't AoE tank, it simply means that they will hold from healing aggro on up to about 6 enemies if played correctly, Shattered Halls can be "AoE" tanked by a warrior. Do not expect to Seed of Corruption or Arcane Explosion your way through a run, however also do not expect them to fail with more than one target.
Warrior utility is to their tanking, they provide a 2minute +1000 health buff to their party, a 20% slowing of the enemies attack speed (this won't generally be noticeable to the healers, but does add up over a fight) and other miscellaneous buffs that can increase attack power and such like, however in general these are applied by a second warrior.
Progression tanking is normally performed by Warriors because of their active response capability, shield wall reducing damage by 75% (so a warrior with 60% reduction from armour has effectively 90% reduction in damage), and last stand granting them +25% health for a short period of time, these tools let them survive in situations that would have killed a Druid or a Paladin, both of whom do not have active response tools like this.
By manipulating their stances Warriors can become immune to fear every 30s, this means they rarely require fear wards, and are the tanks of choice for bosses that fear (Nightbane, Archimonde).
Finally warriors have spell interrupts, this allows them to be placed solo against caster enemies with spells that must be interrupted, the cooldown is similar to a rogue's.
In T4-5 content a Warrior will typically have the second highest health pool, the lowest threat generation and the slowest ramp up. In T5-6 content their threat generation will equalise or tend to become higher and their health drops to the lowest. Warriors tend to have the most avoidance gear in general, however suffer from a relatively hard avoidance cap at which point rage generation drops significantly, in content they over gear warriors have the 2nd easiest time maintaining threat with druids coming first.
Druids have a strange place in tanking content having the highest armour by far (reaching the armour cap of 75% damage reduction), and in T5-6 content the highest health, their tanking tree is also their dps tree however meaning that when present with a Paladin or a Warrior it is often better for the raid to let the Paladin or Warrior tank than the Druid as neither of the other types of tank provides the same DPS capability as the Druid.
Druids threat generation is generally the second highest or highest in the game, with a large proportion of it easily maintained while not tanking (making them the ideal off tank in most cases), as with Warriors the threat tends to come in steady spikes on the global cooldown making threat quite linear.
Druids excel in tanking melee enemies, however the lack of spell damage reduction and inability to avoid crushing blows makes them situational tanks in general, this is not a put down, in a raid typically tanks are / should be few Warriors and Paladins and many Druids due to their ability to fulfil many roles fluidly. Fights like Morogrim Tidewalker, Halazzi and Nalorakk (troll form) are the ideal fights for Druid tanks, high melee damage and large spikes that take effective use of their large health pool and armour.
In terms of utility Druids are a strange class, presenting a +5% melee crit bonus normally, and bringing Blessing of the Wild, a combat resurrection, and the capability to heal (lightly, but an AoE heal never hurts), DPS or Tank at short notice.
In short, if in doubt stack some Druids, their utility to the raid is very high even if their actual tanking can be situational due to the deficiencies of the other two tank classes when not tanking in tank gear (if they can change before the fight Warriors and Paladins can provide a role, but in tanking gear it is much reduced).
Paladins are a strange tanking class, they excel in AoE tanking situations, however in T4-5 content have the highest threat generation and potential in both AoE and single target situations, in T6 the relative gap drops significantly due to badly scaling abilities. Paladins are also mana based tanks, this means they can begin front-loading threat onto targets on the pull without waiting for rage to build up, however they need to take sufficient damage to maintain their threat cycle of roughly 1500 mana / 10s (15,000 damage over that period, or 1,500 dps), interestingly Paladins have the softest avoidance cap as they can be provided good mana regeneration using mana potions, Shadow Priests or Shamans, yet they tend to have lower avoidance in general than Warriors due to gear optimisations.
Paladin threat generation comes in cycles of roughly 8-10s bursts with a lower generation between, the speed of the attacker also comes into play as reactive threat provides 15-30% of overall threat (about 20% on your average boss fight). This means that DPS will often move close to a Paladin's threat then have him move ahead a fair amount, making it potentially harder to predict DPS timings (odd considering its a relatively fixed cycle of threat generation).
The lack of utility and responses is clear also in terms of their tanking, when all is going well Paladin tanks are great, however when enemies get away all Paladin abilities are on long cooldowns relatively (15s for taunt, 3-5minutes for Blessing of Protection). The ranged taunt is a blessing and a curse, it allows the Paladin to reacquire their target without moving, however in so doing DPS must leave it alone until they re-establish threat, hitting it while it is returning will result in the Paladin losing threat again partway through a taunt cooldown.
Paladins completely lack active responses to tank death situations, Lay On Hands is one of the best near-wipe recovery tools (and generates roughly 45% of the tanks health in threat), allowing the tank to heal to full health. Additionally there is typically a passive "shield wall" available below 1/3rd of their health, however this can be skipped by hard hitting bosses and cannot be relied upon to trigger.
On fights with threat resets or debuffs Paladins are often the tank of choice for burst threat and self-removal of the debuff, similarly for AoE tanking encounters such as Mount Hyjal. In general if it can be tanked by a warrior it can be tanked by a Paladin, fears and silences are generally their worst encounters (Azgalor, Mother Sharaz, Maiden), however in general these issues can be worked around relatively easily.
As a final note, Paladin threat generation is also the most flexible, with the addition of a Shaman, Shadow Priest and Retribution Paladin to the tanking group it is possible to increase the tanks threat per second by (threat+70)*1.1. That is often a near 200 threat per second increase if required, with the Shadow Priest and Shaman providing the Paladin with the ability to tank in even low damage situations (a shadow priest being equivalent to roughly 600dps on the paladin, a group of 3 shadow priests can thus theoretically keep up a full threat rotation with no damage taken, this situation rarely occurs but if required can allow for very high avoidance tanks maintaining high threat generation).
In terms of boss fights Prince, Illidan, Nalorakk (troll), Blood Boil are all excellent fights for Paladin tanks.
The three tanking classes are generally all useful to have in a raid, Warriors provide the baseline and "gimmick" tanking capabilities to the raid with Paladins providing the highest threat and AoE tanking capability. Additional tanking is generally best provided by stacking Druid tanks in the raid as this reduces the overhead of stacking protection classes who lack utility while in tank gear. On each boss fight its best to evaluate which tank will be best for that fight and to assign them to it, Warriors and Paladins will keep up their debuffs irrespective of the current tank, if given the chance a Warrior will make a reasonable DPS, or a Paladin a strong, but lacking stamina healer.