Base-Breaking Character: Yuji Sakai after merging with the Snake of the Festival. While he has clearly has well-intentioned goals, others consider him to be an incredibly ruthless bastard who doesn't care about anything else outside of his goals.
The third and final season has split the fandom between supporting Flame Hazes and Crimson Denizens (and in this case, Rooting for the Empire is quite understandable) and between whether the season is an interesting storyline with a good ending for the series or a badly written arc with mismanaged characters and whose main conflict feels pointless even in-universe.
Geneon Dub VS Funimation Dub has fans and detractors.
Yuji's Darker and EdgierAnti-Hero tendencies over the course of season 3 has left many fans grumbling about the drastic change in personality.
The problem stems a lot from the anime adaptation. Yuji expresses despair over the endless, ultimately nihilistic Flame Haze-Denizen struggle very early in the light novels, as well as a growing determination to do whatever it takes to end it. The Snake of the Festival, who shows up out of nowhere with no foreshadowing or buildup at the beginning of Shakugan no Shana Final, approaches Yuji from within the Midnight Lost Child and tempts him with promises of ending the eternal conflict. He also starts to exhibit a much more aggressive personality far sooner as well. For example, in the anime, when Wilhelmina attempts to kill him to prevent the Midnight Lost Child from falling into Bal Masqué's hands, Yuji does little to fight back. In the light novels, after Shana prevents Wilhelmina from harming him, Yuji takes the opportunity to try to kill Wilhelmina, and comes close to doing so. In short, Yuji's FaceHeel Turn, for a given value of heel, is not the abrupt switch the anime made it out to be, but a well-telegraphed and logical conclusion to his character growth.
"Common Knowledge": Shana is the prototypical hyper-aggressive, ultra-violent Tsundere who habitually dominates and abuses her love interest. Except shes not. While Shana introduced, or at least codified, a type of Tsundere that is more hot-tempered and more abrasive to her love interest, and while the hyper-aggressive, ultra-violent Tsundere can claim a direct line of descent from Shana, she lacks the borderline abusive behavior (at worst striking Yuji all of three times in the first season— of a three-season showin moments that are played for laughs), tendency to treat their love interests in a subhuman fashion (Shanas early dismissal of Yuji as a thing is due to the fact that, ordinarily, he would be little more than a non-entity as a Torch), and the tendency to dominate or tyrannize said love interests that they have. Shana actually is closer to the classic Tsundere, a person with harsh outward personality, the result of a sociability problem, who gradually reveals a softer and loving side. Her Character Development is really quite remarkable, even when comparing the cold and hostile Shana at the beginning of Season 1 to prickly but friendly Shana of mid-Season 1, let alone the Shana of Season 3, who can no longer even be considered Tsundere.
Designated Hero: To some people, the Snake of the Festival and/or Snake Yuji is a rare antagonist example- he is clearly intended to be a Hero Antagonist who has the noble goal of wants to create a world where Flame Haze, Crimson Denizens, and Humans can life in harmony. But the things he does in pursuit of this goal are rather hard-to-swallow: sacrificing millions of innocent lives, human and denizen alike, in the war, driving Margery to the Despair Event Horizon, and outright kidnapping Shanato become his bride and almost raping her, all while leaving his other love interests Kazumi and Hecate in the dust. Needless to say, some people view him winning and reconciling with Shana as a prime example of an Easily ForgivenKarma Houdini.
Die for Our Ship: Kazumi, especially after Pheles gave her a Treasure Tool which can summon Pheles, but would take Kazumi's life as a price.
Sabrac is also one of the more popular antagonists despite not having been introduced until the end of season 2. He is mostly remembered by viewers for his memorable battle, and many can back him up as the most badass antagonist in the series. There's also the fact that unlike other villains he had a backstory to his motivations and even a rivalry with one of the main characters.
Decarabia, one of the minor villains introduced in season 3, got a lot of attention. Many were amused by his strange character design and his tendency to constantly appear at the most random of times.
Evil Is Cool: Yuji as Snake of the Festival definitely fits the bill; his fans often nickname him as Darth Vader Yuji, or just Darth Yuji for short.
Evil Is Sexy: Yuji as Snake of the Festival has attracted a lot of fans over to his side because he took a massive level in Bishōnen. Honourable mention goes to Bel Peol, with her choice of clothing.
With A Certain Magical Index. Both are light novels published by Dengeki-Bunko that got published around the same time and have the similar concept of magic in a modern day setting, alongside a male protagonist who is partnered with a hot-headed female lead. It also helps that the anime for Index aired at the height of Shakugan no Shana's popularity and even featured many similar cast members.
Also with The Familiar of Zero, Hayate the Combat Butler, and Toradora!, since their female leadsnote Louise, Nagi, and Taiga respectively are Expies of Shana and are even voiced by Rie Kugimiya. Together, they're collectively known as "The Four Tsundere Wonders" by many fans and are often featured in numerous crossover fan art and fan fiction. Even the anime industry in Japan has acknowledged this friendship and used the opportunity to promote merchandise based on the characters being together.
Idiot Plot: Some feel this way about Season 3, in which the Denizens are simply trying to get out the Flame Hazes' hair by going to a world with limitless Power of Existence. The only reason there is conflict is because the Flame Haze army doesn't trust them to behave themselves and stop eating humans. Moreover, the Flame Haze don't even need to just trust the Denizens. Since everyone agrees that the Denizens should stop eating humans, even the Denizens themselves in the end, Snake of the Festival Yuji could have simply decided on his own, right from the start, to make it impossible for them to obtain power of existence in Xanadu this way. Thus, the Flame Haze wouldn't have to take the Denizens' desire to reform on faith, and the conflict would have been entirely eliminated. In fact, in the commentary for the final episode of season 3, Cherami Leigh (Shana's English voice actor in the Funimation dub) speaks of the "Could Have Avoided This!" Plot if Yuji explained everything to Shana instead of disappearing at the end of season 2.
Bel Peol, known as the "Arbiter of Reverse Reasoning" and "The Strategist", lead Bel Masque in the first two seasons. In the past, Bel Peol sacrificed her right eye as part of a plan to retrieve her master's body; later convincing a Denizen into going for a dangerous mission, resulting in his death for her cause. Trying to destroy Misaki City, Bel Peol nearly succeeds in all her attempts thanks to her tactical brilliance and pushes the heroes to their limits. Joining her master, the Snake of the Festival upon his revival, Bel Peol acts as his Dragon and together they usher in the creation of Xanadu, a paradise for Denizens and humans alike, fighting fiercely for it. A ruthless, goal-driven woman, Bel Peol nevertheless showed fondness to her underlings and exceedingly intelligence thinking in the field of battle, proving herself a worthy leader of Bel Masque.
The aforementioned Snake of the Festival or "The God of Creation", is the original leader of Bel Masque. Summoned in ancient times by the Denizens of the past, the Snake tried to create a paradise for them before being sealed in the Abyss. In present convincing Protagonist Yuji Sakai to become his vessel, the Snake retakes leadership of Bel Masque, giving a speech so powerful it gains him the complete loyalty of all the present Denizens. Leading the Denizens in the devastating Second Great War, the Snake oversees the brutal combat to have his body recovered from the Abyss. Returning to Misaki City to create Xanadu, the Snake uses his own body and a member of the Trinity as sacrifices to create his new world. Realizing the Denizen's wish for peaceful coexistence, the Snake completes his mission despite massive opposition and returns to sleep, wishing his vessel success for the future.
Never Live It Down: Yuji is probably going to be forever remembered since season 3 as 'that would-be rapist world destroyer', no matter what positive qualities he has.
Older Than They Think: A non-nonsense girl with black long hair, a bladed weapon, a black cloak and a powerful, deep-voiced demon partner comes to this world in order to hunt down other demons that feed on humans, disguising herself as an aloof highschool student in order to be near her target. She is secretly in love with another Ordinary High-School Student who has a more social best friend and a sweet female classmate with short, brown hair who eventually bonds with the main girl as well, and one of their first adventures throws the guy and his classmate into the loop. Finally, among the enemies there is a sultry, scheming red-haired villainess that leads several of the demons. Are we talking about Shakugan no Shana or the 1989 manga-turned-OVA Makaryuudo Demon Hunter, protagonized by the eponymous hunter Yama? With such a question, the possibility that Yashichiro Takahashi was familiar with that series when he started writing Shana is certainly hard to dismiss.
Paranoia Fuel: At any time in the past, you may have been killed, eaten, and replaced with a copy without you even realizing it. Eventually, that copy can run out of power...
One interpretation is that the more Crimson Denizens (Guze no Tomogara) are in desperate need of resources and thus feeding on humans is their only option, and some Flame Haze are overzealous. The former is backed up by the idea the Bal Masque was founded by a god, thus they essentially should have the same authority as Flame Haze (though they are not contracted with Crimson Lords). The latter is partially backed up by the fact that Margery Daw was originally shown as being somewhat of a Blood Knight and cared nothing but Revenge, thus indicating that there are more violent and less honorable Flame Hazes.
This became a more viable option in the later light novels (and the third season) when it is revealed Bal Masqué's plan, as well as their new leader Yuji's, is to create a utopia called Xanadu where Crimson Denizens will not need to devour humans to sustain their existence, and even coexist peacefully with them. Considering how the series ends, The Empire wins.
Seasonal Rot: For a lot of viewers the first season was well-received and thought of as a great start to the franchise. However, many felt that as the show went on it got progressively worse. Season 2 was criticized for beginning on a filler arc that lasted half the season, and not doing much to differentiate itself from season 1. It was the third season where many felt that Shakugan no Shana began to lose its touch. Many criticisms were aimed at Yuji fusing with the Snake of the Festival and becoming a much less likable character, the introduction of many new characters that had no development and added nothing to the show with some dying in their first fight scenes, and the Idiot Plot involved in the final battle. Not helping matters for English dub fans is the Channel Hop from Geneon to Funimation causing everyone from season one being replaced, and many considered the Funimation cast to be inferior to the cast Geneon had.
"Seinfeld" Is Unfunny: In the mid and late 2000's this was oneofmanyanime that achieved widespread popularity. Newer fans who try to watch this show however don't get what all the buzz was about. A lot of the tropes that this show popularized are now in tons of anime causing new viewers to find nothing special about this show. It is made even more apparent by the fact that one of its contemporaries Toradora! still enjoys widespread popularity and acclaim to this day and Shana herself is no longer the poster girl for the Tsundere trope.
Squick: The Aizens, Tiriel and Sorath, make out with each other. In public. A lot. Even Shana, who'd only learned about the concept of kissing about a day ago, was visibly disgusted by it.
They Wasted a Perfectly Good Plot: Yuji coming to terms with his upcoming Cessation of Existence with the knowledge that at least he won't be completely forgotten when he disappears could've made for an interesting story (even if the story had to go searching for a new male lead after that). Instead, Yuji turned out to be a Living MacGuffin that regularly restores his own Power of Existence.
Wangst: At times it seems like the series would be half as long if Shana and Kazumi wouldn't take so much time with their angsty internal monologues over Yuji, although Yuji himself is no stranger; he plays this trope at the end of the second season!