If you really and truly consider the implications, you realize that quite literally everything in both the canonical show and the Continuum proceeds directly from The Secret. Because of the Secret, Gentle believes with every fiber of his being that earth ponies are "broken". Because he believes, he is willing to do anything to "cure" his daughter of being an earth pony. And because of that quest, the quest to find a cure for his daughter's species, he discovered the Exception and the other magics that allowed him to save Pinkie and Fluttershy's lives. If it had not been for the Secret, those two would have died in childbirth, and everything would have unfolded in a completely different manner.
And on the Fridge Horror side, all the horrors of the Great Work, her tortured existence, the repeated rape of Discord's essence, all those horrors would never have happened were it not for the Secret. Applejack finally begins to realize this in Chapter 44 of Triptych itself, and it's part of what makes her decide that at some point within the next few years, the Secret itself has to break.
From Triptych, in the chapter titled Earth Tones, when Applejack says this ironically, about Pinkie's father and how he reacted to the first Pinkie Party — because given Pinkie's odder abilities, he might actually be right on how the party got funded:
her father... he started kicking her. Because he said she'd stolen from him. Taken money from the family." ... "Stolen. Seriously, Twilight, stolen. Even if she took the money, the rock farm's way out in the middle of nowhere. Where's she gonna spend it? She'd have to be gone for days just to do the simplest shopping. Did he think he was just looking at his own memories the whole time? That he had an earth pony filly who could teleport?
It ultimately turns out that the party supplies were conjured: part of Pinkie's magic allows her to create things — typically items which are related to her talent.
It's confirmed by Word Of Fanfic Author that the current year (as of Triptych) is 1274. Since the abeyance is likewise known to have stretched for a thousand years, that means that there were a tad under three hundred years between the defeat of Discord and the abeyance. Furthermore, it's mentioned that whatever the Nightmare might have been aware of, Luna effectively skipped over the entire abeyance. If you put all that together, it helps explain a lot about Luna's Continuum characterization.
Luna is noted to look down on modern ponies, considering them soft and spoiled. And while the Chaos directly caused by Discord ended with his sealing, the knock-on effects would have persisted for a long time thereafter. The current level of control and security that most ponies enjoy would not have been established until well after abeyance had begun.
Similarly, it's noted that Princess Luna is far more openly pragmatic than Celestia, caring less about appearing to hold the moral high ground and more about getting the job done. In Eris, that kind of pragmatism would have been universal, and Princess Luna has skipped over a thousand years of social development.
Luna likewise reacts far more harshly to chaos than her sister, because the scars of the Discordant Era are far fresher.
Why didn't Celestia realize that Twilight was a victim of Coordinator's smear campaign in school? Because she had an easier answer: Twilight is just emulating her previous life, so she never looked deeper.
Fridge Horror
It's implied in The Bounce Test (and confirmed by Word Of Fanfic Author) that part of Flash's problem with his mark comes from having been unconscious during his manifest: it's why he truly doesn't recognize what his actual talent is. And upon realizing that every time he follows the suggestions of his magic, accidents happen, he spends his life trying to ignore it, believing he's nothing more than a Trotting Disaster Area. Well — by the end of the story, we know Flash's talent is a defensively-weaponized combination of Destructive Savior and Disaster Dominoes: he sets off accidents in order to prevent worse things from happening. And the things his magic triggers saves lives. This means every time Flash ignored his talent, there's a very good chance that ponies got hurt or died. The reaction when that thought locally sinks in isn't going to be pretty.
As the Continuum has developed, it's become increasingly clear just how badly growing up in the Discordant Era has scarred Luna and Celestia's psyches. Even a thousand years later, they haven't fully recovered. Their dialogue in Triptych Continuum A Markof Appeal makes it clear that they view deliberately releasing Discord as the last of last resorts, only to be done if it is down to a choice between releasing Discord on the one hoof and all sapient life being wiped out on the other. And yet the Sisters have released him. That can only mean one thing: Something is coming that will at the very least render the planet uninhabitable, and we have no idea what it is.
As with so many other ponies, Gentle Arrival is responsible for Pinkie's existence: without his intervention, she (and from what he's said, her mother) would have died in childbirth. But at the same time, his meddling may have instigated Pinkie's abuse — and if she ever learns exactly what he did... note For the other three whom we know to be his: Snowflake's father supported his child's drive without ever truly understanding it, Fluttershy's did what they could to help without ever understanding her, and Ratchette's parents just went through a very indulgent number of dishwashers.
And the answer to what happens when Pinkie finds out is 'near-total emotional collapse'.
Celestia can't interface with MOON, only Princess Luna can do that. Which means that in order to raise and lower MOON, Celestia had to call up the memory of the sister she'd lost to Nightmare as she was before. Twice a day, every day, for a thousand years.
And Cadance possibly has it even worse. Every time she ignites her horn or sings to the earth, she has to remember the ponies who died that she might live.
Fridge Logic
"The Secret", the earth pony conspiracy to cover up the true scale of their magic, makes absolutely no logical sense and seems entirely counterintuitive. It's not even necessary to protect earth pony rights, because the simple fact that Equestria depends on earth ponies to be able to feed itselfnote ponydom is so reliant on the Cornucopia Effect that they have barely even figured out crop rotation means that earth ponies don't need to hide the fact their magic gives them defensive options as broad as anything that unicorns and pegasi can do. This is lampshadedIn-Universe when, firstly, Twilight tries to broach the idea that the Secret may cause more problems for earth ponies than it solves, and secondly when Gentle Arrival's Great Work is stated emphatically to stem from the overall belief of unicorns and pegasi that earth ponies are inherently magic-less by comparison to their own tribes.
(and confirmed by Word Of Fanfic Author) that part of Flash's problem with his mark comes from having been unconscious during his manifest: it's why he truly doesn't recognize what his actual talent is. And upon realizing that every time he follows the suggestions of his magic, accidents happen, he spends his life trying to ignore it, believing he's nothing more than a Trotting Disaster Area. Well — by the end of the story, we know Flash's talent is a defensively-weaponized combination of Destructive Savior and Disaster Dominoes: he sets off accidents in order to prevent worse things from happening. And the things his magic triggers saves lives. This means every time Flash ignored his talent, there's a very good chance that ponies got hurt or died. The reaction when that thought locally sinks in isn't going to be pretty.