Izumi. Does she really have Nagatsuki City's best interests at heart, or is she just a power hungry Manipulative Bitch hellbent on destroying Kyoukai no Kanata? A strict but loving Parental Substitute to Hiroomi and Mitsuki, or a horrible one for nearly getting her brother killed? Was she against Mirai wanting to sacrifice herself to stop it and try save Akihito, or was she glad to rid herself of two powerful threats with one stone? And so on.
Yayoi Kanbara. A harmless Bunny-Ears Lawyer just trying to make her son happy, or a terrible mother with hidden motives of her own?
Ass Pull: Many accused Mirai's resurrection at the end of the series to be this, as it happens suddenly and with very little explanation. It gets slightly averted in I'll Be Here, considering she has no memories. Akihito also mentions it must've been one last act of defiance by Beyond The Boundary, or a necessary sacrifice for Mirai to come back to life. Possibly both.
Audience-Alienating Premise: As some critics noted, Beyond the Boundary was an exceptionally bizarre choice by Kyoto Animation to try to turn into their next big success. The light novel didn't even win the competition it was written for (it ended without a winner, so the novel and other three were given a honorable mention) and it was clear that the author wasn't even interested in continuing the series (the last volume to date was published in 2013, curiously the same year as the anime's run, and it didn't close any major plot thread). People who knew this in advance were understandably wary of the anime.
Base-Breaking Character: Mirai is considered a bad heroine by some, with her inconsistent attitude and her klutziness being out of place cited as the main factors. On the other hand, others find her portrayal a refreshing protagonist who doesn't fall into one of the many expies common to anime. Though many from the first side were cleared after episode 10 (see Rescued from the Scrappy Heap below).
Opinion on the ending is divided, since many fans are either pleased that the series ended on a happy note, or angry since they perceive Mirai's last minute resurrection as an Ass Pull. The movie expanding on this, as told in the point below, was another breaking point.
The I'll Be Here - Mirai-Hen movie. Many people enjoyed it, and it was widely felt as bringing back the quality to the show that was believed having been lost by the ending of the series (see Win Back the Crowd below), but people would have liked to see explained the Ass Pull of the anime's ending, as the film basically pushes forward the plot without giving a clue of why it happened.
And then is the debate about whether the reveal of Miroku and Izumi's yomu nature explained as being possessed by a special kind of youmu. While some think it was adequate and interesting, other would have preferred a less "mundane" explanation.
Ensemble Dark Horse: Ai is widely liked even by people who aren't fans of the show.
He's Just Hiding: A popular feeling some fans have towards Miroku Fujima since the sequel film, as it didn't explicitly show him as gone for good, and some have theorized the mummified corpse isn't actually his and possibly leaves his fate to future installment.
Ho Yay: Hiroomi's habit of teasing Akihito can be interpreted as this.
Episode 7 when Mirai (while freaking out) speculates who Akihito has in his apartment with him. He denies any name she says but when Hiroomi's rolls around he doesn't say anything.
Akihito jokes Hiroomi is falling in love with him, when he worries about how the Calm will affect Akkey in episode 8.
In episode 9 Hiroomi seems to be the only one who actually puts an effort into saving Akihito from being killed by Mirai. He goes as far as to yell at Izumi while he is usually very quiet around her.
Excuse me, are we forgetting that in the end scene of "I'll Be There" Hiroomi glomps the crap out of Akihito. It's memetic!
Episode 5 focuses mostly on Mirai and Mitsuki hanging out together. It should be noted that the entire time she was with her, Mitsuki was blushing. Also, this.
The Mini Theater shorts take the Mitsuki/Mirai Ship Tease up to eleven, with Mitsuki constantly referring to Mirai as "cute", and at least two episodes end with the girls going out on a "date" together.
In I'll Be Here, Mitsuki seems more heartbroken over Mirai's amnesia than Akihito is.
The ending scene of I'll Be Here, when Mitsuki excitedly runs towards Mirai (who now has her memories restored) with tears in her eyes. Mitsuki then proceeds to glomp her and lovingly press her forehead against Mirai's while hugging her tightly. KyoAni wasn't even trying to be subtle with this one.
In episode 8, Izumi gets up close and personal◊ with her secretary as she prepares to test a barrier on her.
Narm: Episode 12. After Mirai vanishes, Akihito lunges for her ring, the only thing that's left of her. After a moment, he lets out a very corny-sounding yell as he tumbles down. Needless to say it kind of ruined the Tearjerker moment just a few seconds before.
Seasonal Rot: Most fans agree that the anime as a whole is at least So Okay, It's Average, but the ending of the final episode and everything that came afterwards left a sour taste in many fans' mouths. The movies and its own series ending at least were received with relief for explaining something of the ending, but they came with their own complaints that other than that, they solved basically nothing.
Squick: In Episode 7, we get a short scene of Hiroomi cutting out a picture of his sister's face and placing it over the picture of a idol singer's body.
Surprisingly Improved Sequel: Even with its varied flaws, the I'll be Here - Mirai-Hen movie is considered this towards the series.
They Wasted a Perfectly Good Plot: There are many plot points (some introduced in the last episode!) that go unresolved or are never explored in depth, among other things because the whole light novel series never did anything with them. Examples include:
Fujima's motivations for wanting to destroy the world. It's handwaved in the movie, but fans still wanted a less mundane explanation.
Akihito's mother implies that there's something special about Akihito's birth, but she never explains it.
Hinted at in the movie, when Yayoi claims to be the gatekeeper between the human world and the spirit world and was apparently close friends with Mirai's mother. It's possible she may have willingly become a vessel for Beyond the Boundary.
Despite this, by the time the movie ends it's still unclear exactly what Yayoi Kanbara's role is and whether her claim of being the gatekeeper between the two worlds is even true or not.
What exactly the Spirit World Warriors Society is, why they are supposedly opposed to the Nase clan, and whether they were supporting Fujima or if he was a rogue agent.
The identity of the old man in the cave under the Nase Clan mansion.
These will probably remain unanswered to those who do not read the light novels if they've gotten to resolving these plots, as the anime doesn't actually follow the light novels very closely.
There were a lot of viewers who weren't big fans of the series until they saw Episode 10.
Post-series, Episode 0 and I'll be Here - Mirai-Hen has gone a ways in addressing complaints about the main show. Including providing a much needed explanation behind the backstory of Akihito's traumatic dream and the incident where he nearly killed Hiroomi, following up on the Broken Base ending and expanding upon Mirai's resurrection, and resolving the romantic plot between Akihito and Mirai.
