What is the Great Curse?
During the Primordial War, the Exalted succeeding in slaying several of the very creators of reality with their magics. Yet as the Primordials died, they cursed their murders, their very lasts breaths casting a terrible rot upon the gods and their Exalted lackeys for the act of slaying their rightwise masters. These slain Primordials entered the Underworld (being too vast and incomprehenisble to enter the cycle of Reincarnation) and hovered near Oblivion while those Primordials that surrendered were banished to Elsewhere and became the Yozi.
At first this curse did little but slowly mature and grow within the Exalted. However, as time went on, the curse finally ripened and began to take its affect. ALL of the Exalted received the Great Curse, but the Solars received the brunt of it. The Great Curse worked to pervert their virtue and their loyalties, transforming goodness into corruption, valor into vainglory, piety into self-righteousness, and pride into arrogance. As the Solars experienced this most heavily, it was they who showed the greatest signs of insanity and corruption. As of such it was they who were killed by the Dragon Blooded and Sidereals "for the sake of preserving Creation".
Even in the Second Age (where the game takes place) the Great Curse still exists in all Exalted. While the Solars still have to deal with it, the fact they aren't in positions of power to abuse and have so much good works to do, helps to protect them against the lashback they might otherwise experience.
In the simplest of terms, the Great Curse is exactly that. A curse, whose job is to corrupt the Exalted and make them suffer for the sins they committed against their Primordial gods.
What is the Great Curse NOT?
The Great Curse is not some sort of power that rips control from the character/player. The character does not feel some power tear a hold of their bodies away from them and puppet them around to commit terrible or pathetic deeds of evil or cowardice. If that were the case, the Exalted would have recognized long ago that they were cursed by something. Rather, the effects of the Great Curse feel natural if somewhat extreme. While the Solar, after recovering from a Limit Break, might admit that "perhaps he went a little overboard", especially if he caused some sort of harm, he will be of the view that it was necessary or at least a reasonable (if, again, slightly excessive) course of action.
For example, someone with the Great Curse Virtue Flaw of "Berserk Anger", when they experience Limit Break, won't have a moment of fear and scream "Oh no! I am losing control of myself and am attacking any who approach me!" as they proceed to hack away at whomever is close by. It will be more like "That is the 8th time that guy has purposefully spilled his beer on me! One day i'm gonna..." *snap* "THAT'S IT MOTHER F*%KER! TIME FOR SOME TROUBLE!"
Additionally, a Limit Break is not horrifying. At least not to the Solar. It is cathartic. A sudden release of pent up stress. As the chosen Limit Break will not be "out-of-character", it really will fit in with the character's personality when it occurs. Someone who is almost always calm if somewhat proud, is unlikely to take Berserk Anger as their Limit Break. They are more likely to take Foolhardy Contempt, which inclines them to challenge anyone that seems to doubt their prowess. This goes a long ways to explain why no one is easily inclined to believe there is some sort of curse upon the Solars. It is just a systemic flaw of the breed (and hence why they were usurped once upon a time).
Great Curse game mechanics.
The Limit Break meter has 10 spots. When those 10 spots are filled up, a Limt Break occurs.
There are 3 ways to fill up the Limit Break meters:
1) The Solar spends a temporary willpower to stop themselves from acting on their primary virtue.
Ex. A solar with Valor as his primary virtue spends a willpower to let himself run away and join his companions rather than fight.
2) The Solar spends a temporary willpower to resist unnatural mental influence.
3) A certain condition is attached to the virtue flaw the player selected. Whenever the player experiences that condition, they must roll a number of dice equal to their primary virtue. Every success adds to the Limit Break meter.
Once the Limit Break occurs the player still controls the character, but the character begins to act in the fashion described in the Virtue Flaw. There is an option to "partially control" your Limit Break (which has an associated cost) so that you don't act quite as crazily. However, I have a house rule regarding that.
--House Rule-- If you wish to retain partial control, you must pass a willpower test at a difficulty of the Flawed Virtue.
During an uncontrolled Limit Break, you receive a number of temporary willpower equal to the Flawed Virtue, even if this takes you above your normal max. This is one of the only times that you can do this.