Alternative Character Interpretation: Fuuma Monou after becoming the Dragon of Earth. The anime and movie portrayed him as a sadist who is more willing to destroy humanity while in the manga, he's less fiendish and did spare a few people only because it's their wish to live. According to Kakyou, Fuuma is only granting the people's wishes meaning if a person wants to die, then he would do it. This is also in line with Fuuma's wish which he states that only Kamui can fulfill. In the anime and movie portrayals, Kamui suspected that Fuuma's shift in personality was caused when he became the Dragon of Earth. However, in the manga, it's implied that Fuuma may be acting of his own free will regardless of being the Dragon of Earth which makes his motives more muddled.
Awesome Art: As accustomed of CLAMP, the artwork can be incredibly beautiful.
Broken Base: About whether the series will (or should) ever continue or not.
Given that X/1999 had been put on hold several times until its official hiatus in 2003 due to mirroring Real Life incidents, fans generally found this understandable; however, Nanase Ohkawa originally mentioned that they would continue the manga when they have a magazine that is willing to publish it. Yet by 2023, decades had passed and CLAMP had gone on to work on a vast number of other manga to completion (Tsubasa -RESERVoir CHRoNiCLE- and its World Chronicle sequel, ×××HOLiC and Kobato., with Tsubasa notably being more than twice as long as X as published) as well as several other projects (Cardcaptor Sakura Clear Card most notably, along with ×××HOLiC: Rei and Gate 7), all without word about X returning. By that point, with decades behind the last release, most fans had accepted that the manga would not be finished and that the "hiatus" had become a quiet cancellation, though some still held out hope for a revival and thought the story should at least be concluded in some way.
The situation is especially unpleasant considering that a lot of X fans were, shall we say, not really the target audience of Tsubasa and xxxHolic, leading to particular frustration about Tsubasa getting so much focus for its run with X being completely ignored. Conversely, however, with the mixed reception of the endings of both Tsubasa and xxxHolic, you have some fans who do not want CLAMP to finish X, worrying that CLAMP's apparent difficulties in getting their post-90s work to stick the landing would result in a similarly disappointing ending for X after all this time.
Diagnosed by the Audience: Satsuki vastly prefers computers over interacting with other people, and seems to be annoyed when people try to interact with her. She also has a distinct Lack of Empathy as shown by her interactions with Yuzuriha. This leads some to wonder if she is on the autism spectrum.
Poor, poor Kotori. A lot of shippers see her as an obstacle in the Fuuma/Kamui ship just because she is Kamui's childhood friend. Never mind the fact that she's dead.
And if you cross paths with a Seishirou/Subaru fangirl, don't mention Kamui. Similarly, saying the name "Subaru" in front of a Fuuma/Kamui fan can be as unwise as mentioning Kotori's.
Epileptic Trees: Because of the hiatus, many fans keep speculating on Fuuma's and Kamui's true wishes. Most believed that Kamui's wish is either to die by Fuuma's hand or let humanity be destroyed while others think Fuuma's wish is for Kamui's happiness or to free him from the burden of choosing humanity's fate.
Ensemble Dark Horse: There are a lot but two most popular are probably the energetic Sorata and the cold yet kindhearted Subaru.
They are probably a lot of fanfics which had a lot of What If? scenarios such as, 'what if Kamui joins the Dragons of Earth instead and Fuuma go to the other side vice-versa'
Though they are fanfics of Kamui visiting Yuuko's shop, CLAMP made a short comic of it in Clamp in Wonderland EX in which Mokona jokingly says that the price of Kamui's wish will be the fate of the world and Watanuki scolds him for making Kamui cry. Fuuma appeares later on and declares that he's the only one who can grant his wish; cue the two fighting again much to Watanuki's chagrin.
Faux Symbolism: This is essentially the Gospel According to Nanase Ohkawa. The major alteration from Judeo-Christian eschatology is the dualism, with the two sides evenly matched and only a slim moral advantage on the part of the Dragons of Heaven.
Ho Yay: C'mon, it's CLAMP. The most prominent case Kamui and Fuuma survives even as they become enemies. There are other common matchups as well, including Subaru and Kamui (mind penetration and Big Brother Mentorship) and Fuuma and Seishirou (mostly for the symmetry, it seems). Seishirou and Subaru are far enough out of subtext and into text that Ho Yay doesn't really apply anymore.
Too Bleak, Stopped Caring: The manga strays into this in the last chapters. The Dragons of Heaven are in a losing battle where Yuzuriha suddenly disappeared, Subaru defected to the Dragons of Earth after inheriting Seishirou's job with the belief that he wanted to help Kamui realize his "true" wish and Arashi became a pawn to Hinoto's evil side who is responsible for the mishaps that the group's experiencing. It also turns out that Kamui's "true wish" is not restoring Fuuma back to his original self. It doesn't help that the manga then went a decades-long hiatus with a Cliffhanger which drove readers mad. Most older readers want CLAMP to continue the story, but due to CLAMP's later works gaining a reputation for having trouble sticking the landing, some care less about it with the fear that the ending is probably be more depressing than even the movie adaptation.
They Wasted a Perfectly Good Character: The presence of Nokoru Imonoyama and the former CLAMP School Detectives in the X canon would have produced some interesting storylines had CLAMP given them a bigger role than mere one-time benefactors. Nokoru's status as the head of the wealthiest family in Japan (and, perhaps more important, their relation to the Magami clan) could have believably have him involved in the conflict in the Dragons of Heaven's behalf, Suoh Takamura would have been a Badass Normal asset, and Akira Ijyuin's role as the master thief 20 Faces would have come handy during the robbed Sacred Sword arc. Instead, they become glorified cameos, and the anime even directly ignores them.
Uke: Kamui and Subaru. While it isn't a BL series (and neither is Tokyo Babylon), the framing leans very heavily on this.
The Woobie: Kakyou, Kotori, Subaru, Kamui, Yuzuriha when Satsuki decides it's time for a very literal Kick the Dog moment and kills Inuki... a good chunk of the main cast, really. Not so strange when you consider that they're Team Keep Humanity Alive. Among the fandom, Subaru is the undisputed King of Woobieness.
Kakyou Kuzuki is a Dragon of Earth. Team Kill Humanity to Save the Earth isn't entirely without Woobies, you know? Outside the movie, pretty much everyone is a Woobie or just plain Affable. Fuuma's theme is granting wishes (read: Killing people who want to die), the Sakurazukamori assassin is a position apparently passed on by death at the hands of the one the current assassin loves (Seishirou killed his mom), Satsuki was used by first a father and then corporate businessmen who want to exploit her. Kusanagi is perhaps the only one on the Dragons of Earth who takes his side (though halfheartedly) because he's a nature-lover. (Oh, and he's ready to Back Stab FUUMA to save Yuzuriha.) Nataku hardly has a past at all because he's a clone. That leaves Yuuto, who's incredibly cheerful and friendly who's pretty much in it For the Lulz, and even then he wasn't fully evil as he has a sister he misses and genuinely cares for and refuses to take advantage of Satsuki's crush on him when he could've perfectly done so.
Nataku's case is ambiguous, since he might not have a proper past since he's a clone but in the manga what's left of the person he was cloned from's memories play a very important part in his very spoileriffic and sympathetic death. You could even make a case for the anime, as Fuuma absorbs Nataku's body to heal himself after Sorata almost kills him because of his Wish and former life's memories, in a sense.
Subaru was a Woobie before X even started, having lost his twin sister at the hands of their best friend / mutual love interest, Seishirou.
Awesome Music: The movie's theme song, "Forever Love" by X Japan which fits perfectly for the last part of the movie where Kamui mourns for Fuuma's death while cradling his head. Do remember that this is the same song played during funeral of hide, the guitarist of X Japan, who died in 1998.
Anti-Climax Boss: Fuuma is killed rather fast due to the movie's time constraints.
Broken Base: Even although most fans prefer the manga and/or anime over the film adaptation, it has its fans due to its grittier, gorgeous animation (some think it is still better than the often Off-Model style of the anime), compressed story and emphasis in action (which eliminated the trouble of the Kudzu Plot which was quoted as one of the low points of the series) and rotund ending (especially for those who dislike Fuuma).
Questionable Casting: In the Spanish dub, Aleix Estadella as Nataku and Kusanagi was considered an incredibly bizarre cast choice, especially in the second case, as he is known for playing funny bumbling guys and neither of the two is two miles in range to fit this description.
Too Bleak, Stopped Caring: The movie can be this since this was released while the manga was ongoing. The world is about to end as usual but both sides ended up killing each other and dying at the same time. The Dragons of Earth are portrayed as mass murdering assholes which includes Kusanagi, who is known to be Token Good Teammate and a Friend to All Living Things. The Dragons of Heaven are still the good guys but they are slowly decimated along with the barriers they're protecting, which resulted to entire city completely destroyed with Tokyo Tower left standing. And in the end, Kamui is forced to kill Fuuma by beheading him which leaves him the only survivor in the cast.
Narm: The abundance of Off-Model animation can seriously disrupt the mood.
Questionable Casting: Aleix Estadella, who oddly voiced Nataku in the movie, was put to voice Sorata in the Spanish dub of the anime. While this was considered a much more fitting character for him, who specializes in bumbling, comical characters (he voiced Jar Jar Binks, to put a quite illustrative example), some critized the choice for making the character sound just too silly.