The Analysis page now has a full breakdown of the entire Gappy Stu archetype under The Gappy's Journey: Just what does DitR deconstruct?, featuring a case study of 55 fanfics of 100k words in length, a breakdown of the character and ensuing storyline, and how it applies to Brolli.
Diamond In The Rough takes into account the fact these children are leaving behind their school and home lives for Gensokyo, leaving in their wake disappearances and case where returnees are driven insane.
Episode 15 finally dives into the answer to the question, "What would happen to a kid who leaves the real world for Gensokyo on a long enough timeline?" The answer: he'll have nothing left in the real world, and the consequences for leaving will only get worse and worse.
Most kids who encounter Yukari are unfazed by her. Here, the encounter with Yukari shows how utterly terrifying it would be to meet an all-powerfulborder demon, as well as bring out the Faustian undertones of Yukari's offer to bring a kid to Gensokyo.
Most Touhou fanfics have the outsiders beat up a youkai on their first encounter, even though youkai are canonically immune to physical attacks unless by magical weapons or individuals. When Brolli tries to fight Rumia (who is later revealed to work for Yukari to corral Gappies, which is why she's often the first youkai they encounter) with a rock/unarmed, the results don't go so well. The rest of the movie hammers home the fact that if you're an unarmed human in Gensokyo, you are screwed.
In most Touhou fanfics, the outsider is flown around all over Gensokyo with no problem, meeting numerous characters and drinking lots of tea. Here, the trip is frustrating Brolli: he is scared of heights, not fond of tea, and each location he visits is rife with danger. By the time he actually does get his powers, he's nearly willing to go back home until he becomes Drunk with Power.
invoked A pattern among Gappy Stus is that they are lavishly dressed, are usually well-to-do, and their parents were never present. This movie explores the psychology of the kind of kid who would accept going to Gensokyo: somebody so shallow and bored/dissatisfied with life that they'd trade it for a dangerous fantasy world. It also brings up (and eventually answers) why Yukari would pick these kinds of boys in the first place...
In Touhou canon, Nitori is characterized as neurotic and slightly arrogant. In fanon, Nitori's sole purpose is to make inventions for anyone who asks, regardless of what she is doing at the time. While Diamond In The Rough sticks very closely to canon interpretations for its characters, Nitori is portrayed as her fanon interpretation for 90% of the film. She finally switches to her canon self in the climax, and it is jarring.
In most Gappy fics, Reimu ends up training the Stu and they become automatically good at danmaku. Here, Brolli, who already has overpowered beasts, still wants to train with Reimu, and he gets his butt kicked. After training is over, Reimu voices how much she (canonically) hates training and gets all of her experience just hunting youkai.
While many Gappy fics have the main character's waifu fall in love with the main character with no problem at all, it turns out here that Brolli's love interest, Reisen, is just cripplingly lonely and desperate for love, even if Brolli is using her.
Later on, when they have sex for the first time, it's portrayed as awkward and uncomfortable, unlike the countless fanfic examples of Idealized Sex.
Even later, it is revealed that Reisen is pregnant, which is a consequence of having unprotected sex a lot that is often not brought up in Gappy Stu fics.
In most fics, the main character is gapped in by Yukari and then forced into allegiance with the Hakurei Shrine. Over time, Brolli grows so tired of Yukari and Reimu's constant instructions that he defects to the Moriya Shrine and the House of Eternity.
Aya often interviews the Gappy as an excuse to show off their powers, but it's revealed here that she interviews people so she can expose their weaknesses. While this is canon, it's broken down even further when it sparks a war.
While unintentionally deconstructed, Diamond In The Rough accidentally brings out just how shallow and selfish Marisa really is and how destructive adopting her lifestyle could really be. Before the audience realizes that she limits her powers to preserve the balance of Gensokyo, Marisa turns out to be Not So Different from the Gappies herself, and the only reason she's still around is she has a track record of resolving incidents and she never oversteps her boundaries.
It also deconstructs the Mighty Whitey trope, present in a lot of gappy fics. In those fics, the outsider is usually a white, male, Western and affluent character that is bored with life and gets dragged into Gensokyo and, in record time, it gets better in danmaku than the canon characters and saves the Japanese realm. In DitR, Brolli doesn't get better than the others and even gets his life in trouble in the beginning, and, only in the last minutes, he manages to accomplish something.
Canon Mokou is very quiet, shy, reserved, and mainly escorts people without saying a work. Fanon Mokou is portrayed as chatty, eccentric, and willing to do anything you ask of her. Diamond In The Rough goes with Fanon Mokou, only to tear her apart when, in Episode 12, it becomes apparent that she just doesn't care: since she's immortal, she does whatever to stave off boredom, and consequences mean nothing on a long timeline.
Flandre's reputation, both in Touhou fandom and in Diamond In The Rough, is that she's violent, insane, and willing to kill at the drop of a hat. We later find out Flandre's reputation is fake: despite being immature, having a morbid sense of humor, and underestimating her powers, she's a normal resident of Gensokyo who gladly fights by spell card rules. She even voices her disappointment with the fact everyone only sees her as an evil maniac, just before showing everyone she can be awesome as a good maniac. The expectations regarding Flandre extended beyond just the story itself since many viewers were caught off-gaurd by the fact Flandre fights by Spell Card Rules: some fans even decried the fact that Flandre should have remained a homicidal maniac and ignored spell card rules, thinking that to be closer to canon. Spaztique even had to link viewers to source materials to say, "Yes, this is canon."
In a more lighter Deconstruction, we actually get to see how destructive Flandre might be in an actual fight where she keeps underestimating her powers. It destroys a large chunk of the mansion, but nothing unreasonable. In fact, she calls out the beasts when they deliberately started taking out the mansion's supports for going too far.
There's an abundance of fics where Yukari gaps in outsiders, and yet Reimu and Marisa have never questioned why so many brats have been gapped into Gensokyo, rule over the locals, and then just die or disappear. Here, after being pushed over the edge when Yukari accuses Reimu and Marisa for both doing nothing about the gappies and aiding them, Reimu and Marisa decide enough is enough and they'll stop Yukari from gapping in any more kids once and for all.
Episode 18 delves into what it would really be like to be an inexperienced kid fighting the "biggest incident in Gensokyo's history". Instead of becoming the big hero, Brolli is thrust into the middle of a horrific war zone, surrounded by death and gunfire in all directions. By the end of his first battle, he is visibly shaken. Worse, he never actually resolves it himself: Miko does it off-camera while he is hiding in an abandoned building.
Episode 19 answers what would happen if an outsider continued taking and taking resources from a certain place in Gensokyo, specifically the Kappa Valley, just before an incident: the Kappa Valley is left defenseless.
The climax plays the God Mode Stu trope for utter horror: by the final battle, almost all of Brolli's allies are willing to shoot the survivors of the Kappa Valley in order to save Brolli. Yukari tells Brolli he can easily beat Tenshi single-handedly, and that if he wins, he will become Gensokyo's new Big Bad. Rather than win, Brolli chooses to die fighting Tenshi.
Throughout the film and in Touhou canon, it's a wonder why Yukari is friendly towards human, yet one of the most devious youkai in existence. Then, we find out what happens to all of the humans she gaps to Gensokyo once she's done with them: massive meat locker, full of all of the children she's ever gapped in.
Brolli tries to pull of a Heroic Sacrifice, but it doesn't work like in the normal fics where it gives everyone a 100% happy ending (and a chance to be revived): although Brolli saved everyone by tying with Tenshi, everyone is still scarred from the incident. The only character who properly feels sorry for Brolli's death is Sakuya: the only person who's ever really seen Brolli trying to improve.