The Curse of Flesh is a curse suffered by some of the descendents of Azeroth's seed races, notably the dwarves and gnomes.[1] The curse changed the structure and appearance of these races from their original stony/metallic forms into the fleshy forms we know of today.
The Curse of Flesh is revealed to have been created by the Old Gods early in Azeroth's history to "facilitate assimilation". The titans attempted to remove the curse (and the Old Gods), but found that the Old God infestation had grown too severe to remove without completely destroying Azeroth. Instead they bound the Old Gods within Azeroth.
Direct mentions of the Curse of Flesh
Brann Bronzebeard
A dialogue triggered by Brann Bronzebeard in Ulduar and Halls of Stone gives most of the details of the curse and the connection to the Old Gods.
Fizzcrank Fullthrottle
In the quest The Mechagnomes, a dialogue with Fizzcrank Fullthrottle details the removal of the curse from several gnomes by Gearmaster Mechazod which transformed the gnomes into "mechanical beings". The dialogue mentions that all creations of the titans eventually fell victim to it.
Sjonnir the Ironshaper
Sjonnir The Ironshaper mentions the curse when engaged.
Therazane
Therazane mentions the curse during her gossip text for the quest At the Stonemother's Call , while pointing that those afflicted with it killed her daughter, Theradras.
Indirect mentions of the Curse of Flesh
Discs of Norgannon
While the Lorekeeper's Knowledge in Uldaman doesn't mention the Curse of Flesh by name, it clearly refers to the same events that are later described by the Tribunal of Ages. Interestingly, this dialogue was written during the original World of Warcraft, indicating that either this plotline has been planned for many years or has been built out of existing lore.
Jotun
Jotun's title suggests that he is cursed, or bears a curse to others (which could be believed to be the Curse of Flesh).
Archavon
In Archavon's Log (before Patch 3.3.0), for the Day 1051213, Archavon wrote "Archavon wake up, notice face feel soft. Not important. Go back to sleep." This could hint that he was, more or less, affected by the Curse of Flesh.
General Bjarngrim
While General Bjarngrim in the Halls of Lightning does not directly utter the name of the curse, he does refer to it during the fight. His quotes imply that he compares himself with the player. Apparently, he also considers killing lifeforms affected by the curse to be the the solution of removing it.
Lady Deathwhisper
In Icecrown Citadel, some of her dialogue states that the Curse of Flesh is a joke played by the Creators on their own creations. She also states that the sooner the cultists come to terms with the 'fact' that the Curse is a defect, the sooner they will come into a position to transcend it.
Afflicted races
Earthen/Dwarves
The Tribunal of Ages dialogue reveals that it was the curse that caused the transformation from stony earthen to fleshy dwarves while they were asleep in Uldaman.
Stone troggs/Troggs
The troggs too, were initially stone creatures. Like the Earthen, some were affected by the curse, turning them into fleshy troggs.
Mechagnomes/Gnomes
If Gearmaster Mechazod is to be believed, prior to the curse, gnomes appeared as mechagnomes. Unaffected mechagnomes have been found in the Storm Peaks, maintaining the mechanical systems of the Titans.
Vrykul/Humans
Sjonnir the Ironshaper appears as a vrykul with metallic skin. As such, it was thought possible by some players that the vrykul also had an original, more stony, appearance and also suffered from the curse. However, the 'iron' races were constructed by Loken, after and separately from all other titan creations. Instead, based on the quests 
[71] The Echo of Ymiron and [71] Anguish of Nifflevar, it is thought that the birthing of humans may have been originally a severe side-effect of the curse on the vrykul.
Aesir, the storm giants
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This article or section includes speculation, observations or opinions possibly supported by lore or by Blizzard officials. It should not be taken as representing official lore.
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The storm giants of Northrend may be aesir who have succumbed to the curse. Aesir is the titan word meaning "storm giant".
Tol'vir/Ramkahen
The tol'vir of Uldum are a race of stone cat-centaur introduced in Cataclysm. The curse of flesh causes them to turn into the fleshy, wingless Ramkahen. The process can be reverted by powerful beings such as Siamat, and is a some kind of a 'reward' for swearing allegiance to Deathwing.
Earth elementals
The curse of flesh can effect the various earth elemental types within Deepholm. This causes them to turn into creatures akin to fungal giants.
Pygmies and Ettins might have formerly been some type of earth elementals, too.