Broken Base: Red Mist'sAlas, Poor Villain moment in Volume 3. Some feel it does humanize Chris somewhat, and he get's a Redemption Equals Death hammered in by the fact that even though he's sorry, Hit Girl dismisses his apology because of all the harm he's caused. Others feel it's a cringeworthy attempt to try to ring out sympathy for a truly hateful character, and tries to bank more on his film portrayal than the monster he's always been in the comics.
Darkness-Induced Audience Apathy: Anyone who seems to be in the right is quickly revealed to be naive or secretly an asshole; the main character is a spineless worm who's deluding himself into thinking he can be a hero, while the primary hero character is an utter tool who has turned his daughter into a violent criminal to live out his own dreams. And yet the people they fight against are even worse. There's a reason a lot of people prefer the movie's significantly more idealistic take on the story.
Foe Yay: Red Mist being... Red Mist: "...and if I'm absolutely honest, I've wanted to see you in pain since the first night we met. I've even jerked off about it. Does that sound weird?" Yes, Red Mist. Yes it does.
Red Mist and his father manage to cross it at the same time, after setting up Kick-Ass and the others. First, they beat the shit out of them. When Hit Girl tries to fight back, Johnny G orders his men to shoot her in the back, which they gladly do, sending her flying out a window. Red Mist then raves about how awesome that was, demonstrating that he's either a total sociopath or at least isn't very good at separating comic books from reality. In either case, that's the moment when both of them stop being mere villains and cross the line. Interestingly, the film turned this into a humanising Even Evil Has Standards moment for Red Mist by having him instead be suitably horrified.
Also the revelation that Big Daddy isn't an ex-cop and dragged his daughter into the lifestyle after running away from her and lying to her about the fate of her mother. This is one point where a decent amount of people seem to prefer the film version of the character over the comic due to the absence of this.
In Volume Two, they pull out all the stops to make the villains seem unstomachably cruel, clearly in an attempt to eradicate any surrogate thrill the reader might get from their exploits. Don't-give-a-shit wanton violence is such a primal power-fantasy, they really wanted it to seem as disgusting as it would in real life. Most common criticism is that they went too far in this, parts of the book are just really unpleasant to read.
Dave Lizewski wants to become a real-life superhero. After a few weeks spent walking on walls and wearing his costume under his clothes, he decides he wants to start fighting crime for real. His first attempt to do so ends as well as you might expect with him getting beaten up by a trio of vandals, stabbed, and finally getting hit by a car. Ultimately he manages to survive and recover from all of this, and although he tries to give up the superhero lifestyle, he soon gets drawn back into it. You'd Expect: That if Dave was insistent about being a superhero, he'd try and take some martial arts/self-defense classes, or do something to give himself more chance in a fight, as soon as reasonably possible. Instead: He does absolutely nothing of the sort until the next series.
The Motherfucker wants revenge on Dave because of the latter's role in the death of his father. He decides to go after the people Dave cares about. You'd Expect: The Motherfucker to try and limit his victims to those connected to Dave, and not cause unnecessary bloodshed. Instead: While going to attack Dave's crush, he and his supervillain friends massacre an entire suburb, respectively killing and injuring about 30 and 100 innocent people, including children. This results in the Motherfucker's friend Vic Gigante withdrawing police protection from The Motherfucker. And then, he decides to start another massacre in Times Square, seemingly for no other reason than it sounding cool. Ultimately, this course of action leads to his downfall, and his actions turn him, and his secret identity, into figures of public hate.
Pointed out by Kick-Ass in Volume 2, after he & Hit-Girl find out that the Mother Fucker's plan is to burn New York to the ground - Chris is asthmatic, so what's he going to do when he blows up all of the pharmacys & hospitals, and he needs his medication?
The Woobie: Dave and Mindy suffer a whole lot through the story. Angie Genovese becomes this in Volume 3 where she becomes a social pariah for the crimes Chris did.
Crosses the Line Twice: A lot more than twice actually. Single scene example: Dave gets stabbed in the gut. The audience winces. He then staggers out into the street and gets flipped by a car. The audience starts laughing.
During the torture scene, Dave reflects on the fact that he would miss out on seeing what he and Katie's kids would look like. However, in Kick-Ass 2, they break up early in the film.
During Dave's opening narration, he mentioned that since his mother died of Aneurysm in the kitchen, the viewers won't be expecting an "I will avenge you mother!" storyline. This is exactly what happened in the sequel when Chris/The Motherfucker had his father brutally tortured to death.
He Really Can Act: Nicolas Cage surprised a lot of people with his portrayal of Big Daddy. It helps that the movie character is much more sympathetic then the comic book character.
Before Big Joe burns Big Daddy during the torture scene, he says "For all of you cavemen out there, this is called fire" then it cuts to Todd and Marty watching. In 2013, Nicolas Cage (Big Daddy) and Clark Duke (Marty) voiced cavemen in The Croods. In the film, fire is indeed an important plot.
It Was His Sled: The movie makes no secret of the fact that Red Mist is The Mole, whilst it was a twist in the comic.
Red-Mist to alesser extent. He's much more sympathetic in the movie in comparison to the comic.
This will evidently continue in the sequel, as Red-Mist as Motherfucker's more violent acts are going to be removed, particularly the rape, to which Christopher Mintz-Plasse flat-out said "Thank God." He's still pretty evil in the sequel though as he murders Dave's father, kills Colonel Stars And Stripes, and attempts to rape Night Bitch (and when that fails, he puts her in the hospital).
The scene of Big Daddy testing the bulletproof vest on Hit-Girl by shooting her with a Beretta 92FS became a popular staple of meme comics because of how disturbing it is out of context.
Moral Event Horizon: D'Amico is most firmly established as a complete psycho-ass when he murders both some guy unfortunate enough to be wearing a Kick-Ass costume and an innocent witness to said murder. He did believe it was the real Kick-Ass and that he'd been killing D'Amico's men. He really crosses the line when he finds out Kick-Ass had nothing to do with it but still insisted on him being beaten and executed on a live webcast. The sheer look of sadistic pleasure on his face and the disgusted look Chris gives him when they're both watching it said it all.
One-Scene Wonder: Mr. Bitey had probably less than 30 seconds of screen time, and is still one of the most remembered and oft-quoted parts of the movie.
Rescued from the Scrappy Heap: Pretty much everyone due to the movies making the characters actually sympathetic rather than a bunch of jerkasses. Of particular note are Big Daddy and Red Mist.