And why you won't see many of them.
Before we start the discussion, i emphasize that the warrior class is the exception to the rule. Being designed by Blizzard to both tank and deal damage the class is a hybrid class. But what i'm here to discuss is why paladins and druids cannot and will not tank in any sensible guild. And why shamans, priests, paladins and druids will rarely if ever be accepted into the dps role.
The way Blizzard designed raids and stagnated progress back in the day was a combination between players being new to the game, and the fact that 40 man bosses dropped 2-3 pieces of loot. That meant, a single MC run with it's 10 bosses would drop on average 25 pieces of loot. In order to equip the whole raid, you needed around 620-640 pieces of loot (depending on Dual wield, off-hand and shields etc). So the raid required a tremendous amount of farm, and the second boss in BWL was almost a pure gear check, were nothing but farming MC over and over satisfied the requirement. So guilds set up a raiding rooster where each and every piece of loot advances the guild.
Paladins, the worst dps class in all of vanilla. Some argue that they were somewhat decent after gearing up in AQ40/naxx gear vs the undead bosses in naxx, and i tend to agree. Paladins where in fact pretty damn useful with the AQ40 set, but the remaining pieces had to come from somewhere and that would be the vastly superior warrior class. Healing paladins equipping their off-set gear they got when nobody else needed the loot can do decently in Naxx. But for a guild attempting to enter BWL will quickly realize that their paladin dps is utterly useless in dealing damage. A paladin using ''Nightfall'' can be argued for, but in reality it's much better to make a warrior equip it and spam hamstring, as it's cheap to cast and it counts as a hit so it procs the debuff on the weapon. The only reason why a paladin would be actually hitting the boss would be on fights were the paladin constantly spams cleanse, so having him melee the boss with SOC on while spamming cleanse is better than standing around doing nothing.
Priests are far more useful than paladins. Not because of their damage, even though it's not terrible, but because of the debuffs they bring for the warlocks. ''Shadow weaving'' increases the shadow damage the target takes and is very useful for the raid indeed. But because of their terrible damage, there is no real reason to give the priest any gear before warlocks or mages, who deal much more damage than the priest. Getting back to the 2-3 pieces of loot per 40 man boss, warlocks and mages are simply the better choice when assigning loot to a guild doing progress.
Druids have some useful buffs as well. But once again, there is no reason to give leather gear to a druid when you have vastly superior rogues in the guild who need the loot more. Balance druids are the exact same but they contribute spell crit instead. But there is very little leather gear suitable for spell damage that a Resto druid, shaman or paladin might need for healing. So take them for the increased crit, and keep in mind that paladins return 100% of mana spent when crit healing.
Enhancement shamans have the useful wind fury totem, but it doesn't stack with rogue poisons so it's not terribly useful for the melee. So you're only really taking them for the melee warriors. Their damage is better than that of the paladins, but not much and hunters deal much more damage than shamans again. Elemental shamans are in the same boat as balance druids, there really isn't any gear that resto shamans cannot use.
So to sum it up:
Retribution paladin - none for progress. They are utterly useless outside of naxxramas.
Feral druids - 0-3 for the melee groups. But giving them any gear is pointless, rogues benefit way more.
Balance druids - 0-3 for the spell crit. Most importantly mages, warlocks and paladin healers.
Shadow priests - 1-2 to keep the shadow damage debuff up and running. Give them gear after mages and warlocks.
Enhancement shamans - 0-2 for the wind fury talent bonus, but make sure they equip the Nightfall axe for increased spell damage. Increasing their hit/crit will increase the up time of the Nightfall debuff.
Elemental shaman - None for progress. They are useless.
Basically you should treat hybrid dps as non-healing supports. They give you debuffs/buffs, but they themselves deal horrible damage. So for a guild doing serious progression, they should be on a separate loot table. Behind the pure dps classes, but in front of off-specc and disenchant.
Before we start the discussion, i emphasize that the warrior class is the exception to the rule. Being designed by Blizzard to both tank and deal damage the class is a hybrid class. But what i'm here to discuss is why paladins and druids cannot and will not tank in any sensible guild. And why shamans, priests, paladins and druids will rarely if ever be accepted into the dps role.
The way Blizzard designed raids and stagnated progress back in the day was a combination between players being new to the game, and the fact that 40 man bosses dropped 2-3 pieces of loot. That meant, a single MC run with it's 10 bosses would drop on average 25 pieces of loot. In order to equip the whole raid, you needed around 620-640 pieces of loot (depending on Dual wield, off-hand and shields etc). So the raid required a tremendous amount of farm, and the second boss in BWL was almost a pure gear check, were nothing but farming MC over and over satisfied the requirement. So guilds set up a raiding rooster where each and every piece of loot advances the guild.
Paladins, the worst dps class in all of vanilla. Some argue that they were somewhat decent after gearing up in AQ40/naxx gear vs the undead bosses in naxx, and i tend to agree. Paladins where in fact pretty damn useful with the AQ40 set, but the remaining pieces had to come from somewhere and that would be the vastly superior warrior class. Healing paladins equipping their off-set gear they got when nobody else needed the loot can do decently in Naxx. But for a guild attempting to enter BWL will quickly realize that their paladin dps is utterly useless in dealing damage. A paladin using ''Nightfall'' can be argued for, but in reality it's much better to make a warrior equip it and spam hamstring, as it's cheap to cast and it counts as a hit so it procs the debuff on the weapon. The only reason why a paladin would be actually hitting the boss would be on fights were the paladin constantly spams cleanse, so having him melee the boss with SOC on while spamming cleanse is better than standing around doing nothing.
Priests are far more useful than paladins. Not because of their damage, even though it's not terrible, but because of the debuffs they bring for the warlocks. ''Shadow weaving'' increases the shadow damage the target takes and is very useful for the raid indeed. But because of their terrible damage, there is no real reason to give the priest any gear before warlocks or mages, who deal much more damage than the priest. Getting back to the 2-3 pieces of loot per 40 man boss, warlocks and mages are simply the better choice when assigning loot to a guild doing progress.
Druids have some useful buffs as well. But once again, there is no reason to give leather gear to a druid when you have vastly superior rogues in the guild who need the loot more. Balance druids are the exact same but they contribute spell crit instead. But there is very little leather gear suitable for spell damage that a Resto druid, shaman or paladin might need for healing. So take them for the increased crit, and keep in mind that paladins return 100% of mana spent when crit healing.
Enhancement shamans have the useful wind fury totem, but it doesn't stack with rogue poisons so it's not terribly useful for the melee. So you're only really taking them for the melee warriors. Their damage is better than that of the paladins, but not much and hunters deal much more damage than shamans again. Elemental shamans are in the same boat as balance druids, there really isn't any gear that resto shamans cannot use.
So to sum it up:
Retribution paladin - none for progress. They are utterly useless outside of naxxramas.
Feral druids - 0-3 for the melee groups. But giving them any gear is pointless, rogues benefit way more.
Balance druids - 0-3 for the spell crit. Most importantly mages, warlocks and paladin healers.
Shadow priests - 1-2 to keep the shadow damage debuff up and running. Give them gear after mages and warlocks.
Enhancement shamans - 0-2 for the wind fury talent bonus, but make sure they equip the Nightfall axe for increased spell damage. Increasing their hit/crit will increase the up time of the Nightfall debuff.
Elemental shaman - None for progress. They are useless.
Basically you should treat hybrid dps as non-healing supports. They give you debuffs/buffs, but they themselves deal horrible damage. So for a guild doing serious progression, they should be on a separate loot table. Behind the pure dps classes, but in front of off-specc and disenchant.