Vathara may have perfectly explained a canonical key plot twist in two ways without realizing it; she says the Dai Li joined Azula because "Fire Is Loyalty" so she was able to "Bend Their Minds" with her "Inner Fire", but she also showed another explanation; "Earth is Enterprise." The Dai Li suffer greatly to defend their city from evil spirits, and prior to Long Feng's thought police scam they were honored as protectors. People feared them for their supernatural scars, but they also respected them. Once Long Feng made them into Mind Raping bogeymen, they never got respect, only increasing amounts of fear - people only saw them as the monsters who kidnapped and brutalized their friends, turning the women into those freaky Joo Dees because their efforts to fight evil spirits were suppressed because their prevalence is related to the War, which is illegal to speak of. In effect, Long Feng kept them from getting paid. They were permitted, perhaps even compelled by Earth Kingdom ethics to blow him off for a better deal.
Agent Bon: We give up everything for our city. Can't it give us something back?
Katara, Sokka and Aang were pretty ignorant of the cultural differences. Given that their contact wiith other people is mostly "fly on Appa, land for sleep and food, take care of any problems we find then leave", it makes sense. They weren't purposely ignoring the differences until it was pointed out. They just didn't stay in a place long enough to learn.
The whole "flying on Appa" makes a difference, definitely. In chapter 72, Katara gets angry when faced with Zuko and says how he's safe up where he is while his people are not down the mountain - the gAang have a tendency to see things from the high ground rather from where the real trouble is.
It comes out in-story that the name of Sozin's wife was Fire Lady Tejina, which was in fact a false name for the dragon Makoto. Made absolutely perfect when you realize that 'tejina' is Japanese for "sleight of hand", while 'makoto' is "truth".
Chapter 79 all but states the problem everyone in the world has with the Fire Nation, but lack the cultural attitudes to understand. Due to their natural state of intrigue and warfare(as well as Hot-Blooded instinct), Fire Nationals become extremely vague and formal whenever they feel pressured, as the slightest ill-conceived word could ruin formal agreements or even get them killed. In other words, they lawyer up. And Only Bad Guys Call Their Lawyers.
"(Zuko) grew up in court," Karasu stated. "Nephew. You're among kin."
This explains why Raava is so violent whenever she has sole control over the Avatar State. As another troper pointed out in the Legend of Korra pages, some of this is saving/freezing Aang helped screw up the world's balance and her attempts to fix it reflect that (especially against Ozai). However, how is Aang is more peaceful than the spirit of light/peace/good? In Embersverse it does make sense. Raava follows the dragon's code. Make that, the dragons (subconsciously) follow Raava's since they're from her world. Humans aren't, so it makes sense that the 12 year old airbender kid is more of a pacifist than the Spirit of Peace and Goodness.
Other Beginnings and Embersverse explanations: Humans arrived on the lion turtles. Spirit bending becomes Fire/Earth/Air/Water with appropriate teachers. Vaatu breaks the barrier between worlds. Why do the turtles have the bending? Maybe because the humans fear giving the great spirits power over them (spirits almost wiped out humans, it took Wan two years to find that Air Turtle, yet he found Water and Earth within a year) or they're disturbed by the way the elements force the benders into line with their element's values and the possibly fatal consequences of breaking of those bonds.
Wan has gold dragon child eyes. The soldier we saw possessed by the aye-aye spirit was transformed fairly quickly. Yes, Wan might have just lasted so long possessed but not fused with Raava while fighting Vaatu because she'd entered him so many times to switch elements, but maybe his dragon blood helped him somehow, since dragons are of Raava's world. Also, Raava needed to hold the other three elements for Wan because she's the Spirit of Good, not Agni/Tengri/etc, she couldn't make him a yaoren because that's not what she's Great Spirit of.
Sokka being the one to beat Ozai. Given the Ozai looks down on those who aren't firebenders and those who are Fire Nation, and is willing to throw away his family if he has no use to them and cares only for his goals and ambitions, Sokka, in a sense, is a physical representation of what Ozai disdains, a non-bender from outside the Fire Nation who is devoted to his family and honestly cares about people. Given how Ozai dislikes non-Fire Nation people, Sokka's adoption by Temul probably makes him one hell of an insult in Ozai's eyes (or would if he knew). Add in the fact that earlier in the story, Sokka was the one that pointed out it doesn't have be Aang who kills Ozai, Katara made a comment a boomerang. Sokka beating Ozai was foreshadowed!
This fic originally had Tombcannon quite annoyed. It's loaded Fire Nation apologetics, used to paint literally the whole Fire Nation - every single one of them, even Sozin, as innocent victims of a ruthless Avatar Kyoshi off on a Roaring Rampage of Revenge. At the same time, Katara and Aang were mysteriously transmogrified into Idiot Ball carriers with one-dimensional, mind-numbingly ridiculous takes on life that were completely contrary to the personalities established over Season One's canon (which the fanfic claimed to be adhering to). Throw in Dragon!Zuko the fire-water dual-wielding reincarnation of Kuzon, erase every single bad decision he made in canon and make him a self-flagellating noble hero who can do no wrong, add awesome into everything Zuko does and have the multitudes of Original Characters taking his side, reminding the reader in stylized "monologues" of exactly how fantastic Zuko is, hide behind a variety of excuses ranging from everyone being an Unreliable Narrator to "There are no excuses, only reasons" (suspending this rule if your name is Zuko or Iroh or you're Fire Nation)... this is officially the most brilliant satire/trollfic of all the hundreds of less well-presented fanfics with similar plotlines. Realizing all that, then going back and reading Embers from the start... Vathara is a genius. A genius..
Fridge Horror - While Xiangchen kept Yangchen shut away from the world, this allowed Skylord Subodei, his son Yisugei, and his son Subodei (and let's not mention the rest of the White Wind clan) to conquer, rape and pillage all around the world. The White Wind being the equivalent to the Mongols, think about what Genghis Khan had done. This is most likely why there are still traces of airbender blood in people who are NOT Air Nomads.
It gets even worse when you realize that following a Way is the air equivalent of giving loyalty to a lord, making a deal, or being part of a tribe except WORSE because lack of air will kill you within minutes instead of hours so if you don't have someone immediately on hand to breathe for you there's no chance of survival. Meaning that anyone who follows Xiangchen's Way, willingly or not, is in the same position the Fire Nation is in because of Kyoshi's decree. No airhealers to help them survive and considering Gyate didn't know until it was too late that it could kill her there's a very good chance that nobody would know what to do if it happened. All those kids growing up in the Air Temples would have no idea that going against the Way they've been taught would kill them. If the Avatar wasn't except from this Aang would be dead by now.
In a way Embers runs on this trope. Not the reader's horror, but Vathara's. For instance: remember when we see Iroh and Zuko sailing away from the North Pole on their raft, and Zuko finally getting rest? An almost happy moment, they are reunited, Iroh finally gets Zuko away from bad influences, and he gets to rest. But wait, there's more. Clean water to drink—for now there's the ice bergs but they will run out before they hit land. Food, there's none on the raft, in Embers they ate the equivalent of seagulls that landed on the raft. Plus they had just seen an entire fleet get smashed in the space of seconds. Oh, and dead bodies? They tend to float. Yuck.
There's even a few cases of this in-series. Two of the more common ones are when people discover just what it was that Kyoshi did to the Fire Nation and when they discover just what Aang did at the Siege of the North Pole. Death at the hands of a spirit? Death without any possible attempt to fight back or while full of strong negative emotions? Improper burial (those who drowned at the North Pole would have been irretrievable, and even if they could be retrieved, Northern Water Tribesmen either wouldn't know or wouldn't believe that Fire Nation dead need to be cremated or given a funeral light)? Any of these can cause a soul to be restless. The Siege of the North caused a whole army to experience all three fates. That means there's an army of demons and restless souls wandering southward along the bottom of the sea... not to mention that the angry Ocean spirit is going to keep spitting up monsters of its own to express its rage at their presence.
Fridge Logic - Zuko claims the murder of Katara's mother was a crime even by Fire Nation law, and Katara is justified in seeking vengeance. But that fire bender wasn't there to kill Katara's mother, he was there to kill "the last southern waterbender" - Katara - as per the Fire Nation waterbender extermination policy. Becomes Fridge Brilliance because Katara's mother wasn't a water bender; intent means nothing for the Fire Nation, thus the killing was unjust. If that's the case, then the realization that killing Katara would have been A-OK in the Fire Nation's eyes - that Kya's sacrifice saved her twice over - would most likely drive her Survivor GuiltUp to Eleven.
Nuanced in Chapter 60, where the Fridge Logic disconnect between Puppetmaster (where waterbenders were captured, not killed) and The Southern Raiders (and the death of Katara's mother) is Lampshaded: Fire Nation policy was to capture waterbenders. Either the orders changed for some reason, or Yao Rha decided he didn't want to bother. In which case, it was straight-up murder. (Not to mention, defying his orders.)