The mindless state is a state of mind that brings the afflicted to a point where their mind is no longer their own, and they become mindless zombies or savages. It is referenced in game by Gretchen Dedmar, who claims that her time is coming to an end and she will soon lose her mind. It is seen within older Forsaken, or those whose minds have been damaged from the rot and decay, or by their raising from the dead.
A very similar state seems to afflict the worgen, even being referred to by various NPCs of Gilneas. While not specifically called the mindless state, the two have striking similarities and can both be defined as quite literally mindless states.
Forsaken Mindless State
The Forsaken mindless state is described as one slowly losing their mind and becoming a mindless zombie. Gretchen Dedmar describes feeling a cold chill from the Plague of Undeath, as a sign it is to soon be upon her. Once the mindless state has been reached, she states she will no longer be able to aid the cause of the Forsaken. The Mindless state seems similar to a form of senility for the Forsaken, however much more severe.
Given the nature of how Zombies are within the game, mindless lumbering beings who serve no useful purpose outside of combat (and even in combat they lack skills that other Undead possess), they are very likely the result of a Forsaken who has succumb to the Mindless state, or were raised into it due to their minds having been too damaged. Zombies within the Lich King's service fit the description, often doing nothing more than acting as fodder or managing simple tasks.
The World of Warcraft RPG describes the mindless state within the Horde Player's Guide as:
"Forsaken do not mature into adulthood, an individual becomes Forsaken upon death. While technically immortal, some Forsaken exhibit signs of old age, losing their identities and eventually sentience. They appear as dying old humans, feeble and scared, with a cold growing inside them, and forgetting who they are. They are losing their mind to the Scourge, and would eventually become a "mindless one" - an unintelligent zombie. All forsaken fear this process, as it means they revert to the Scourge's will and lose themselves to the Lich King and his dominance. The Forsaken will often kill these poor souls, perhaps as a form of mercy."
Worgen Mindless State
The Worgen mindless state is much different than that of the Forsaken one, having stages before reaching full effect. Similar to the Forsaken's, the Worgen mindless state seems to only affect those who were afflicted by the Worgen Bite, rather than by drinking the blood of the Worgen. This mindless state however, unlike the Forsaken one, is reversible if treated in time; becoming progressively harder (and eventually impossible to cure for some) if it goes on for too long. During the Gilnean worgen outbreak, it became protocol to put down captured Worgen who had been feral for too long and were unresponsive to Krennan Aranas' treatments.
According to the words of Darius Crowley, the state can be soothed (temporarily) if Worgen are brought to Tal'doren, and can be treated permanently if one drinks from the Well of Tranquility, the Well of Balance and the Well of Fury before the end stages of the Mindless state set in. It seems that the original night elven worgen can be saved from this state as well, as seen by Denmother Ulrica who claims that some of the worgen of Talonbranch Glade (including herself) were summoned when Velinde Starsong first used the Scythe of Elune.
Oliver Harris along with several other Gilnean Worgen have set up a camp at Raven Hill in Duskwood with the goal of saving Worgen in the forest who have not yet reached the mindless state, via alchemy. They have succeeded in saving one man, Sven Yorgen, who had not been Worgen for very long and through force of will had been clinging onto his last vestiges of humanity. Harris refers to this state as the "madness and savagery of the woods."
Once the final stage of the Mindless state has been reached, that person is gone forever, and from the words of various NPCs, no cure can save their mind. They take on a mentality similar to that of a wolf, and will even form packs.
As of the Shattering, the Nightbane pack seems to be the only surviving natural Feral Worgen pack remaining. The Bloodfang pack, while made up of Feral like Worgen, is different from the other packs as the majority of their Worgen seem to retain some intellect and sense of being, which the Nightbane lack.
As the Bloodfang seems to consist solely of whatever remained of Silverpine Forest's worgen, and upon seeing the Northrend worgen, Arugal may have had some method that prevented his worgen from succumbing to the Mindless state, which would explain why only the two groups of Worgen under him show signs of self control, while the others do not.