Badass Decay Averted: While Tai Lung does become kinder, gentler, friendlier, and an all-around good guy and hero, trading his villainous tendencies for snark, he remains as awesome and incredible a fighter as ever—not to mention he’s still quite able to unleash his rage and temper on the true Complete Monsters.
Complete Monster: Both Chao and Xiu take this trope and run with it gleefully. The acts Chao perpetrates are especially, and truly, unforgivable, and they only get worse as the story goes on. Countless murders (including of children), Demonic Possession, More Than Mind Control, forcing friends and loved ones to kill each other, unleashing demons from the spirit world, necromancy, corrupting people's souls and intentions, pushing Tai Lung into committing his rampage, planning to make People Puppets of the whole world, all so as to live forever and force his vision of perfection on everyone else... Xiu, meanwhile, killed her own father just for being "weak" and good, framed Jia for it so she could twist and manipulate her into obeying and serving implicitly as an assassin, longed to have the same powers Chao had so she could do the same things he did, poisoned countless soldiers who had been fighting the Huns, murdered her teacher, tried to manipulate Tai Lung into joining her with lies about his family (and also helped plant doubts regarding Shifu and Oogway's love and pride in him so he would fixate on the Dragon Scroll to earn them), nearly killed Po and Tigress, did kill Zhuang, loved the feeling of Chao's dark chi and wished to bathe in it when everyone else was sickened and terrified by it... And both of them enjoyed every bit of what they did, with no remorse or regrets, and no sign they could be redeemed or ever would be.
Foe Romance Subtext: Heian Chao is just a little too obsessed with Tai Lung and making him his own, whether the snow leopard wants it or not… Apparently it's noticeable enough for even Emotionless Girl Chun to remark on it: "Don't worry, we'll keep [Jia] well away from him. Wouldn't want to interfere with your own plans to have the stud all to yourself."
Chao is the perfect antagonist for Tai Lung. Why? Through corrupting and manipulating his chi, he intends to ‘take Tai Lung’s self away’. Not only is that at the heart of kung fu philosophy, it’s the problem Tai Lung had all along, not believing in himself so that he felt he would be nothing without the Dragon Scroll. Ergo, take away his self, and he’d immediately be back to the monster he was before.
Thanks to the species of the characters involved, any time Tigress is facing one of the Wu Sisters it's a literal Cat Fight. However, every single one of those encounters is taken absolutely serious and is a deadly, legitimate battle rather than fanservice or amusing bickering (although a bit of the latter does precede the Tigress/Xiu combat in the birch forest). The same also holds true for Mei Ling vs. Jia during the Final Battle. Objectification of women defied!
Harsher in Hindsight: According to Word of God, an appropriate voice for Hai the elephant would be Dianne Wiest. This is the woman who, as the Evil Queen of The 10th Kingdom, spoke the line, "If I hear one whisper, one rumor, that anything is amiss, I will kill your children in front of you." This is precisely what happened to poor Hai herself.
When Tai Lung is attempting to 'lighten the mood' on the way to Chorh-Gom by helping Tigress and Shifu reconcile, Tigress has this to say: "And since when have you become an expert on entertainment and leisure? Did you take up clowning when I wasn't looking?" No...but as she'll discover, Ping has◊. (And now that that image is in your head, just try and keep from imagining Tai Lung doing and saying all the things the goose did when Tigress was sick.)
In Chapter 38, Tai Lung's thoughts: "No one needed to know he'd been so caught up in romancing Tigress he might not have noticed if Po had started dressing up in a cheongsam. Oh gods...why did I have to go there? Someone kill me now, please...or at least murder my imagination." Well it may not be a cheongsam, but we have gotten to see Po dressed as a woman in "Ladies of the Shade". And Tai Lung's reaction is pretty apt.
On a related note, Song herself being a violet-eyed snow leopardess who is nominally on the side of the bad guys, performs a Heel–Face Turn to help fight her former allies, and has a Ship Tease with Po sounds a lot like Jia.
The Big Bad is a dark master of chi who used to be allied with Master Oogway, but was eventually corrupted by his lust for power, leading Oogway to seal him away until he finally made his return centuries later, after Oogway's death. Sounds an awful lot like Kai, doesn't it?
The third movie also has a moment where a panda dons Master Flying Rhino's armor (and which Po himself admitted to fantasizing about), and the statues of the Vault of Heroes appearing uncovered and open on the mountain around the stairs... And considering both Heian Chao and Kai are spirits, it's surely the case that Chao was also immune to the Wuxi Finger Hold, which makes Tai Lung warning Po not to use it incredibly prescient. It's a good thing the possibilities he suggested never happened with Kai, isn't it?
Tai Lung: Just what d'you suppose would happen if you used the ultimate chi move on the ultimate master of chi? The least that would happen is it blowing up in your face and doing absolutely nothing. Worse, you might just make him stronger...or allow him to turn it back on you, corrupt you, vaporize you...
Ho Yay: Very much unintentional, but some might see this for Mantis/Monkey and Zhuang/Tai Lung, as well as Tai Lung and Po.
Les Yay: There seems to be a bit of this between Chun and Tigress in the final fight.
Like You Would Really Do It: Somewhat subverted, for while neither Tai Lung nor Po die, a number of instances occur where it looks like they have or will, and such was the skill of the writing that none of the readers seemed to believe the author wouldn’t have the guts to kill them off. This may have been helped by other matters such as the Disney Death of Mantis and the actual deaths of Zhuang and Vachir.
Moe: Nearly every child in the story, including Tai Lung, but Yi especially stands out.
Moral Event Horizon: Though no one was planning to nominate either of them for sainthood, Xiu’s murder of Zhuang and Chao first killing children and then his fellow masters both made them utterly irredeemable and had readers crying out for their blood.
Romantic Plot Tumor (averted): Though Tai Lung and Tigress’s romance is important, particularly to his development as a character and his ability to resist the Big Bad, and quite a bit of time is devoted to it, it was always planned to be so. At the same time, there’s much much more going on in the story than just this, as the length of the main page can attest.
Strawman Has a Point: Most of the naysayers, from Tigress to the various villagers, have rather good points regarding the possibility of accepting Tai Lung back into the Valley, either based on him truly having done reprehensible things which need to be acknowledged and punished or by raising very legitimate fears that such a thing could happen again if he did not learn to control his temper or wasn't truly attempting to redeem himself. While Chang and Xiulan fill such roles, Vachir in particular has a number of points since not only did he lose family members as the others did, he was actually the legitimate authority in charge of imprisoning and punishing the snow leopard. Even the fact that Chang was a misogynist, Xiulan a rather self-righteous and narrow-minded shrew of a wife, and Vachir was revealed to be a torturous bully (and that the latter two both got possessed and manipulated into truly heinous acts by Chao) did not change the ultimate sensibility of some of their points—something which Tai Lung himself acknowledged as he struggled to change, prove himself, and remain true to his new path. Deliberate and intentional by Word of God as part of the fic's Gray and Grey Morality.