In Episode 10, Yukari asks Brolli, "Do you even remember why you came here? Or why you're staying?" Brolli's motivation was already pretty flimsy to begin with, but Brolli's speechless reaction hints he really doesn't know why he's even in Gensokyo.
Audience Participation: The U Stream chat allows for people to interact with Spaztique while he's making the series or with eachother during the screenings, either riffing Brolli's antics or quoting video games.
Cessation Of Existence: The ultimate fate of the consciousness associated to a soul after it has been burned in Hell if the conciousness associated with it has not learned their lesson. This is the fate of Arturo and the implied fate of many Gappy Stus before him.
Crapsaccharine World: Double subverted. On one hand, Gensokyo really is a nice place if you're strong enough to survive in it. On the other hand, everyone is going to kill you if you're considered too strong.
Dark Fic: Starts out pretty edgy, using Touhou canon against the typical Marty Stu OC, but once the incident starts, it hits rock bottom and continues digging until it hits the other side of the planet.
Epic Movie: By both Walfas and actual film standards, Diamond In The Rough clocks in at more than three hours. The final battle scene itself is a little over fifteen minutes and features characters from nearly every game.
Fandom Specific Plot: A Deconstruction of the classic "kid gets gapped by Yukari into Gensokyo, travels all over the land while meeting everyone, finds love, defeats the bad guy during the worst incident ever, and becomes the hero" plot, showing how shallow these plots really are.
Foreshadowing: At the beginning of episode 4, Flandre and Patcholi are studying different literary concepts, such as one that describes the moment a hero's life starts to falls apart. Guess what starts to happen to Brolli near the climax. Heck, Patcholi even is about to explain Foreshadowing before Marisa crashes through the library.
Funny Background Event: Remilia is having a pie when she first meets Brolli. After giving Brolli a tour of the mansion in hopes of persuading him to become her new butler, Sakuya is still carrying the pie around... on her head... and there's a bite taken out of it...
After cutting back to the outside world, the desk next to Jack's reads "Arturo Wuz Here."
When Brolli and Marisa are staking out Reisen, Koishi can be seen talking to some kids in the background, and then disappears upon realizing who he is. You can also see Aya interviewing Kyouko and the Myouren temple when Reisen first arrives.
invokedGod Mode Stu: Deconstructed with Arturo, the previous Gappy Stu. His powers allow him to win any battle, but he doesn't exactly use his powers for good, or at least good in the eyes of others...
Going a step further, there are also some Japanese language jokes that may get lost on non-speakers, such as some of the puns ("Nearby Noodles") and conventions (most of the characters rarely say "no," but instead say, "not really").
Played straight in the full movie when Brolli leaves Gensokyo through the Hakurei Border, only to find himself stranded in Japan, unable to understand Reimu since she's been speaking Japanese this whole time.
Gut Punch: Although Diamond In The Rough starts as a dark twist on the "kid gets gapped into Gensokyo and becomes a hero" story, the incident turns Diamond In The Rough from a darkly humorous poke at Marty Stu Touhou fanfics into something downright malicious.
The Hero Dies: We know from the opening Brolli will die, and the rest of the movie goes into great detail of whether or not he deserved it.
Infant Immortality: Averted. Arturo actually killed several of the human children in Gensokyo prior to Brolli's arrival.
Jumped at the Call: It doesn't take long for Brolli to take up Yukari's offer: he jumps so quickly that he leaves his cat behind.
Justified Title: A diamond in the rough is either an uncut diamond or something good with bad surface traits. It can refer to the original Marty Stu who evolved into a Tragic Hero, the character himself who was an ordinary kid recruited by Yukari to be Gensokyo's powerful new incident resolvers, and it's about a guy with the name Diamondback going through a lot of rough stuff.
Karma Houdini: Marisa even lampshades that she gets away stealing Patchouli's books without any punishment ( too bad that Broli decides to listen to her, instead of Reimu). Also, Yukari definitely is one.
Phantasmagoria Of Flower View: Aya, Komachi, Shiki
Mountain Of Faith: Nitori, Momiji, Sanae, Kanako, Suwako
Subterranean Animism: Yuugi, Utsuho, Rin, Koishi
Undefined Fantastic Object: Byakuren, Nue, Nazrin
Ten Desires: Miko, Mamizou
Others: Suika, Kasen, Tenshi, Lord Tenma (who is mentioned in the supplementary material), Hiraku Okami and Mini from MiniWitch3's videos, entire ARMIES of youkai and fairies, and the Marty Stu that started it all, Brolli Diamondback. There is also Brolli's classmates, including a more prototypical Marty Stu, Jack Diehard.
invokedMarty Stu: A Deconstruction, showing how desperate and manipulative someone would be to want such power and what they'd do with such power. Averted with the actual Brolli, whose conflicts are mostly psychological in nature.
Further deconstructed with Jack Diehard, who doesn't possess Brolli's self-awareness: he is a sociopath a brat who asks Yukari to let Brolli die so he can have a chance at Gensokyo.
Also deconstructed, as we'll come to learn, with Vic: Vic vowed to relinquish his powers and change for the better, but now he cannot readjust to normal life.
Deconstructed again with Arturo, the previous Gappy before Brolli, who only goes to Gensokyo to lose his virginity, even if it means killing children with his new powers, and since nobody can oppose him, he is free to get away with what he wishes. The only solution for the people of Gensokyo is to void normal dueling rules and just kill him altogether.
Mayfly December Romance: Between Brolli, a human teenager, and Reisen, a moon rabbit who is at least fifty years old, telling Brolli she fled from the moon during the Apollo Invasion, to which Brolli reflexively shouts she's old enough to be his mom.
Slasher Smile: Yukari can pull this off, a notable example being her first appearance, which can nearly be considered a Jump Scare.
Sliding Scale of Idealism Versus Cynicism: Starts off just below the Cynical Side, progressively rises to Idealistic Side, and finally takes a long nosedive down the Cynical end for the remainder of its second half until credits.
Translator Microbes: Implied. Everyone in this Japanese wonderland seems to speak English, and Reimu mentions (albeit behind one of Brolli's thought bubbles) that Yukari messes with the heads of the children she gaps in to make the world more understandable. Later in the full movie, when Brolli actually leaves Gensokyo, Yukari's effects wear off, and Reimu speaks untranslated Japanese.
Marisa, according to many preview audiences, is basically a Gappy who escaped punishment: she can gain power and get away with it... But this is a subversion: she's actually The Fettered who loves to look like The Unfettered, but still wants to protect the balance of Gensokyo. Therefore, she never gains more power than she needs.
You Can't Go Home Again: Implied in the first half: After Brolli has stayed a whole month in Gensokyo, Reimu offers Brolli the chance to go back after staying a month. Brolli knows this isn't an option. When Reisen asks if he's glad he left the outside world, he briefly shows a look of disappointment.