I think that class specific spells that require a reagent, should allow that reagent to stack to a count of 20. For the reagents that are farmed drops, this is already the case. I think that some or all of the purchased reagents only go to a stack of five, and for balance could be upped to 20 without effecting anything.
I will try and keep this top post updated as information is shared and discussed, and apologize up front if I miss anything as I do not have lots of knowledge of all of the individual classes and their needs.
Druid: (2 Slots)
Druids have a various seeds for their rebirth spell, which does stack up to 20. If they want to carry a stack of each to down rank they would lose a maximum of four bag slots, but I do not see why you would down rank rebirth.
Druids also require [Wild Berries] for their 30min Mark of the Wild buff, which already stacks to 20.
Hunter: (1 Bag, 1+ Slots)
Hunters may have it the worse as they lose an entire bag slot for their ammunition. This can not be avoided as they would lose an attack bonus that their quiver's provide. I have seen discussed that maybe if their ammo stacked more this would be better or more balanced.
Hunters also use additional bag space for food needed to keep their pets happy.
Mage: (2 Slots)
Mages need [Rune of Teleportation] and [Rune of Portal] for their teleport and portal spells respectively, which I believe stacks up to 10 and should be increased to 20.
They create food and water for everyone, but this is on an as needed basis and disappears after logout.
Paladin: (2 Slots)
Paladins have [Symbol of Kings] for their 30 min blessings, These stack up to 100.
Paladins also have [Symbol of Divinity] for Divine Intervention, which currently stacks to 5 and should go to 20.
Priest: (2 Slots)
Priests need the [Light Feather] reagent for their feather fall spell, which is a farmed drop.
Priests also require [Holy Candle] for their 30min fort buff, which already stacks to 20.
Rogue: (4+ slots)
Rogues lose bag space for their poisons. This is similar to Alchemy and requires either carrying the reagents and vials and mixing them as needed, or pre-making their poisons. I do not know what this takes space wise, but it is several slots.
Rogues lose one bag slot for their [Thieves' Tools] so they can use their Lock pick skill.
Rogues require [Blinding Powder] to use their Blind skill, which stacks to 20.
Rogues require <Flash Powder> to use their Vanish skill, which stacks to 20.
Shaman: (7 Slots)
Shamans lose four spots for their totems. It has been suggested that a quest to combine these to a single totem would be not unbalanced, available at level 40.
Shamans need a reagent [Ankh] for their reincarnation spell, which only stacks to 5 and should be 20.
Shamans need [Fish Oil] and [Shiny Scales] for water breathing and water walking, which already stack to 20.
Warlock: (1 Bag, 1+ Slots)
Warlocks lose out the second most in that they need shards for most of their important spells, and these do not stack at all. At best they lose out on a bag slot similar to Hunters, but are able to store these in a normal bag with no damage penalty. I have seen suggestions of allowing these to stack, maybe up to 5?
Warlocks create Soul Stones and Mana-Candy that can be give/used on other players, which take up to two additional bag slots. (These disappear after logout similar to mage food/water, with the exception of unused soul stones here on TurtleWoW.)
Warlocks require [Infernal Stone] or [Demonic Figurine] to summon Infernals and Doom Guards respectively, but I know nothing about if this is something they usually carry in the hopes of a full group willing to die for the warlock. They both stack to 5 and should be increased to 20.
Warrior: (0 Slots)
Warriors have no class related bag loss. They get punished in other ways in this wonderful game.