When creating a raid, either 10 or 25 man, there are a few things that a raid leader must consider. The number of tanks, healers, and dps is of primary concern, and class makeup, depending on the fight, can also be important. A raid leader must also take into account things beyond a person/class's ability to provide tanking, healing, or dps. Each class brings extra buffs to a raid, which in this post I will be referring to as utility. Utility isn't necessarily a clear cut buff that increases someone's stats like Prayer of Fortitude or totems, but could be a debuff like Curse of the Elements or Expose Weakness.
The main problem with utility is that it does not stack well per class. One Prayer of Fortitude is great, but two is unnecessary. The second Priest in your raid only brings their healing or dps. This is a prime example of the diminishing marginal returns from class stacking.
For an example, if two Priests are CoH specced, then, assuming they are of equal skill and gear, they both bring a value of 5 (arbitrary number) to the raid due to the healing they provide. The first Priest also brings Prayer of Fortitude, which has a value of 2 (arbitrary number), while the other Priest can not provide additional value. In this case Priest A has a value of 7 while Priest B has a value of 5. Bringing Priest A increased the value of your raid by more than Priest B, and thus stacking Priests had diminishing marginal returns. This isn't to say that Priest B is not valuable, simply that two Priests are not worth double the value of one Priest.
Some classes stack very well, while other's do not. In addition to this, specs can play into the marginal utility of a character. If Priest B in the previous example had been specced into Divine Spirit then he or she would have provided additional utility that Priest A could not, though the two Prayer of Fortitudes would still not be valuable beyond the first.
The following is a list of the utility beyond the tanking, healing, and dps that classes provide, listed by class and spec. If you feel I have missed anything, please comment and I will make the effort to correct my mistakes.
Druid: One Druid can provide Mark of the Wild for the entire raid, as well as Thorns for the tanks. Keep in mind, however, that a Druid with Improved MotW and/or Brambles has more utility than a Druid without them.
Balance: Only one Boomkin Aura per party has value, since they do not stack. In addition, only one Boomkin with Insect Swarm and Improved Faerie Fire is valuable to a raid beyond the dps provided.
Feral: Only one Druid with Leader of the Pack per party has value, since they do not stack.
Hunter: For the most part, Hunters are a low utility class whose main value is in the dps and CC they provide. Some specs, however, bring one or two utility ability to the raid that the others do not.
Marksmanship: Only one Trueshot Aura per party has value, since they do not stack. In addition, only one Improved Hunter's Mark per raid has value, since it also does not stack.
Survival: While more than one Survival Hunter will provide increased uptime on Expose Weakness, the increased uptime suffers from diminishing returns.
Mage: Like Hunters, Mages are a low utility class whose main value is in the dps and CC they provide. The class skills that suffer from diminishing returns are Arcane Intellect, Amplify and Dampen Magic, the Mage Table, and ports.
Fire: Only Fire Mages suffer from diminishing marginal returns based on spec, since only one Fire Mage is needed to keep up the Improved Scorch debuff.
I don't want this post to stretch on forever, so I will return to the list later in the week. Until then...
For the Horde and the Naaru!
Correction: Tree of Life aura and Ferocious Inspirations do stack