Eddy's Brother: Was he always a brutal bully to Eddy or did the abuse only start at a later point in his life? Episodes like "Ed, Pass It On" strongly suggest the former, yet several of his Pet the Dog moments throughout the series (teaching Eddy how to make the El Mongo Stink Bomb, promising to give him his car when he turned 14, trusting him with his Porn Stash) point to the latter interpretation.
Eddy: There are several interpretations as to why Eddy chose to seek help from the one person who had been tormenting him for most of his life:
Given how much Eddy still brags about his brother, he could have a warped view of their relationship and genuinely believe his brother would be willing to help.
The movie also retroactively gave this to Eddy's parents, who were thought to be as neglectful as Ed's, given that it's reasonable they may have given him tough love so as to not end up like his brother. Eddy after all was scamming kids. Not to mention, he has a Number 1 Son mug in his room and his brother checks to see if their parents are around, indicating they're protective towards Eddy against his wrath.
Arc Fatigue: A majority of the film covers the kids journeying from the cul-de-sac to Eddy's Brother's house, with next to no actual plot development. Almost everything between leaving the cul-de-sac and arriving at Eddy's Brother's house could be cut out of the film and very little would be lost. If anything, it would just be a half hour episode.
Author's Saving Throw: The movie addresses a few complaints about the show: Kevin and Sarah getting hit by karma, Kankers helping the Eds for once, Eddy apologizing for his actions, and the Eds becoming accepted by the community.
After learning that Eddy's Brother was a cruel Jerkass who tormented Eddy for most of his life and lead to him becoming a scamming Jerkass, watching him get hit with the door of his own trailer is worth a cheer or two.
Cargo Ship: Kevin got shipped with his bike a lot thanks to the movie, where it's an entire plot point that he cares more about it than Nazz, much to her fury.
Just what exactly was the scam that caused the widespread destruction in the cul-de-sac shown at the beginning of the film and how did it go so horrifically wrong? There are some hints to give an idea, but it's made vague enough that it's really up to interpretation. Unsurprisingly, this has led to many theories and fanfics that attempt to piece together the hints and explain what the scam was and what went wrong.
Fanon: While Eddy's brother is never named, Matthew and Terry are one of the common names for him. The latter based on his voice actor Terry Klassen, and the former based on rumors from the staff.
It Was His Sled: Eddy's brother isn't the godly hero he makes him out to be, but a borderline abusive bully who's turned his brother into a fearful follower.
Jerkass Woobie: Eddy, after Edd briefly ditches him. He does become a full-fledged woobie after his brother beats him up and reveals that his jerkass behavior was all a facade.
Misaimed Fandom: Some viewers (with quite the shallow hatred of Eddy) enjoyed Eddy getting beaten by his abusive brother because of the quicksand prank, ignoring that Eddy had already suffered and learned from that experience. Others at least decided when Eddy's had enough and thought his brother eventually went too far.
Moral Event Horizon: Eddy's Brother crosses it when he begins to hurt Eddy for no other reason but his own sadistic pleasure, and then admits freely that he's always done this to him in his whole life.
One-Scene Wonder: Eddy's brother only appears in the last five minutes of the movie, yet leaves a huge impression.
Padding: All of the scenes with the Neighborhood Kids and the Kankers trying to find the Eds. They don't really add much development to the plot in regards to any Hidden Depths or Tear Jerker moments that the Eds provide throughout the movie. As the Ass Pull entry above points out, their scenes could have just been cut and nothing would have been really different. There's also one scene where Rolf investigates Eddy's Brother's destroyed car, even though Rolf already knows it's the exact same car the Eds tried to escape in and there are really no clues that Rolf uncovers that provides any new information about where the Eds are heading.
Eddy. After learning that his brother is an abusive jerkass who frequently beat the living daylights out of him, prompting him to create a Jerkass facade as a desperate attempt to make friends - then recounting various episodes where Eddy is punished and humiliated... who knows what would have become of him were it not for Ed and Double D's companionship?
Edd, after the quicksand prank. Especially with his reaction when he believes both his friends are dead, he seems truly broken. He even said he was thinking he had lost "the only two people I have left in this world".
Jonny. The ending of the movie was happy and heartwarming for everyone...except for Jonny, who is beaten up by the kids, left without friends, and even turns evil. All because of a misunderstanding.