Alternative Character Interpretation: Upon realizing what position Azula is in at the end, many have posited that she doesn't actually have amnesia, as it's all a ploy to get in good with the Water Tribe royalty so she can dismantle the entire civilization from the inside.
Angst Dissonance: The constant weeping, the gloomy dialogue that gets copy-and-pasted as much as the art, and the really stupid spelling mistakes tend to ruin the drama for most readers.
Ass Pull: Ursa's return; we never get any explanation about where she's been or why Iroh waited until that moment to bring her around and no one seems to address it in story at all.
Toph gets this treatment as well, going from a devil-may-care fighter to a soft-spoken, nervous flower of a girl who willingly bottoms to a guy she calls "twinkle-toes".
Katara, too. The things she does the most in this fic? Whining. Being self-centered. Making even her unborn child's death all about her. Only caring about being Zuko's love interest. Wearing pretty dresses (which isn't bad in and of itself, but considering the canon setting, they're very out of place). The only scene in which she's ever seen using the Action Girl skills that she worked so far for in the series? When she bloodbends Mai to death.
Canon Defilement: The names are misspelled, the mechanics of bending are all wrong, the characters are unrecognizable, and somehow we've translocated to the Theme Park Version of 18th century France if costume and architecture are anything to go by.
Designated Hero: Katara. She's presented as vain ("I'm sure that Kuzon will come out quite charming, with me as his mom."), self-absorbed ("[Kuzon] died years ago, a day before my birthday..." emphasis not added), and murderous (do we even really need to mention Mai's horrifying death again?). Yet she's somehow always right and no one ever questions her.
The same could be said for Zuko. Cheats on his wife, fathers a baby with the designated heroine mentioned above, smacks Mai when she confronts him, leaves his kingdom to go to Katara... and yet we're supposed to sympathize with him?
Designated Villain: Mai. She gets mad at the husband whom she's genuinely in love with for having an affair, whereupon he strikes her. We're supposed to hate her, why?
Not only that, but the fact she hid the letters telling Zuko about his child is apparently hideous treason. It's not a nice thing to do, sure, but she had good reasons to do so and it's certainly not treason. Particularly when Katara had every intention of hiding the child from Zuko anyway, and the only reason the letter came anywhere close to Mai or Zuko is a clerical error.
Die for Our Ship: The fandom is comprised almost entirely of Zutara fangirls/Mai bashers looking for a quick fix.
Ensemble Darkhorse: Sho, to an extent. Probably because (a) he's an original character, and therefore does not bastardize any Avatar characters simply by his presence, (b) his "evil" alignment means he's generally trying to kill off said bastardizations of Avatar characters, and (c) he's pretty much the only badass and good-looking character in the comic.
Evil Is Cool: Sho actually manages to pull this off. He comes in out of basically nowhere, nonchalantly tossing around a knife like Mai. Then, he fights Sokka singlehandedly, slyly trash-talking with arguably the most cleverly-written dialogue in the entire comic (not that that's saying much). He almost manages to defeat him, too, until Toph steps in. And then he escapes with a badass firebending move that nobody has seen before.
Evil Is Sexy: Diaz tries to dial this down with Sho as much as possible because he's supposed to be evil. However, interested readers will notice that Sho is more muscular than Zuko, Sokka, and Aang.
From Mai's perspective, it's almost a horror story, as pointed out before: Her beloved husband cheats on her and fathers a love child, but she's willing to let it pass and destroys the evidence. When her husband finds out, he hits and shoves her and abandons her, and his babymama horrifically and painfully kills her.
And now, with the recent development of daytime bloodbending in The Legend of Korra, think about Katara's bloodbending scene. She not only bloodbent Mai in broad daylight, but she used that technique to kill her in front of witnesses. Yes, daytime bloodbending is possible now, according to canon, but it's also highly illegal even when it isn't being used to kill. Sho has solid evidence to convict Katara, not only of his sister's murder, but also of a notable in-universe war crime - but he can't get that evidence into court. Because Katara is Princess of the Water Tribe and now a Fire Lady, with high-ranking friends in every other government on the planet, she is effectively safe from ever having to withstand a fair trial. In the eyes of the law, Katara can do no wrong - what's to stop her from being even more horrible to her subjects than Ozai and Azula ever were? Especially since her subjects are Fire Nation, her tribe's long-term enemies - and she obviously hasn't acquired the same forgiving nature she had toward the end of the series? All hail Fire Lady Katara, folks. And the main characters wonder why Sho's revolution is able to gain ground so fast in the sequel...
When you get right down to it, half of this is Sokka's fault; if he hadn't accidentally sent a letter about Katara's pregnancy to the Fire Naion, Mai would have never suspected anything and most of this could have been avoided.
"Funny Aneurysm" Moment/Harsher in Hindsight: Toph, after hearing that Sokka revealed Kuzon's existence by sending a letter to Zuko, says "Katara is going to kill you... I can only imagine what Mai will do to Zuko...". A demonized, Out of Character, Yandere Mai poisons Katara, causing her to miscarry note it had happened at the time, but Katara was unaware of Mai's involvement at this point in the story, and later tries to kill her herself.
It's astonishing how similar Zuko confronting Mai about hiding the existence of his love child with Katara by hiding her letters until a servant showed them to him is to Ozai confronting Ursa in a flashback in The Search over Zuko being Ikem's son rather than Ozai's- he was wrong but didn't know it at the time, something he learned by the servant Ursa entrusted to deliver her letters betraying her and giving them to Ozai. Zuko may be turning out more like Ozai than he thought here.
Sho's use of firebending to make a disappearing act is also highly similar to something Zaheer did in season 3 of Korra, only he did it with air, a far more transparent element, making Sho's move suddenly all the more plausible.
Except for the fact that most bloodbenders still cannot bloodbend during the day (which is why Yakone only did it during the day, so that people would not suspect that bloodbending was taking place). Yakone's family had a unique special bending ability, much like Combustion Man.
Moral Event Horizon: Mai killing Katara's unborn baby via poisoned fruit is meant to be this. For more objective readers, it's Katara bloodbending Mai to death, when she was perfectly capable of merely incapacitating her so she would be taken to trial for attempted murder.
Narm: Some of the "romantic" or "sad" scenes are absolutely hysterical. Such as Aang glowing avatar-style when having sex with Toph, Azula asking the badly photoshopped duckturtle to be her friend, the photoshop talking panda, etc.
Ron the Death Eater: Mai, who poisons Katara to cause her to have a miscarriage and later tries to kill her. Then again, this might be a subversion, as she's arguably more sympathetic than Zuko or Katara.
Rooting for the Empire: Given how our "heroes" are more or less complete assholes, it's no surprise that most of the comics detractors root for Mai and Sho due to Mai being a lot more sympathetic than Katara and Sho being a Badass.
Zuko confronting Mai about her actions regarding his love child with Katara. It's memorable partly because it's a good indicator of the comic's sense of morality (the guy who had a child out of wedlock is the sympathetic character), and writing (two words: "chardmonster").
Katara bloodbending Mai to death, is fairly well remembered, given that it's not only highly morally questionable, but also a blatant violation of canon.
Snark Bait: There have already been dramatic readings!
So Bad, It's Good: The art is blatantly traced, and often seems completely unbelievable for the situation. The characters are ludicrously Out of Character, and the dialogue is terribly written, but there's quite a few unintentionally funny moments and lines.
They Wasted a Perfectly Good Plot: Just about everything that goes on in the background could have made a more interesting story than the actual story. Aang finding a underground temple of surviving Air Nomads? Glossed over in the prologue. Azula's adventures, which somehow involved her blackmailing Mai to get a seat in the new Fire Senate while simultaneously dressing like a ninja? Never elaborated upon. Sho's existence? Nope. We get to focus on a convoluted and increasingly stupid romance story between six fatally brain-dead people.
Unfortunate Implications: The Sokka-Azula romance only coming around because Azula got amnesia and cannot remember whatever she did. Diaz thinks that the only way these two characters would romantically work out, is if Azula became an entirely different person, who comes across as child-like and immature, leaving the already narmy sex scenes to come across as squicky.
Unintentionally Sympathetic: Mai is supposed to be the straight villain, and does some pretty nasty things, but the comic makes it very clear that everything Mai did is because she is in love with Zuko. Despite everybody else's love being so important to the series, the series itself never treats Mai's feelings as important.
The fact that the "stigma" and "duties" are taken care of by a few choice words by Zuko, also detracts from this as it renders all points that were supposed to garner sympathy meaningless.
The Untwist: The fruit Katara ate was what caused her miscarriage. Despite this being shown in the beginning of Chapter 2, albeit as a flashback-dream, Katara is convinced that it was her longing for Zuko that caused it. It isn't until Mai flat out tells her that she gave her poisoned fruit that made her miscarry, that Katara even realizes it.
Wangst: Most of the couples - particularly Aang and Toph, wangst over how they can't be with the one their love, despite having no real reason why they can't. While Katara's angst over losing her unborn son is understandable, mourning for three years and building a giant tombstone for him is a bit extreme.
It's not just about Katara's extreme mourning, it's about how she makes it all about her. "Kuzon" died a day before her birthday, after all. The dead child is treated as just an extension of Katara's derailed personality and a plot device to have her murdering Mai in the worst way possible.