These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Alas, Poor Villain: Chris, who dies saving Mindy and asking her to apologize to his mother for ruining her life.
Darkness-Induced Audience Apathy: A major and recurring complaint about the comic. Many feel that Millar went so far with the deconstruction that the result was a comic that became downright unreadable at times. This is also a major reason that many consider the film adaptation Better Than Canon.
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.
Jerkass Woobie: Katie in Volume 2. Good lord, did she not deserve that to happen to her.
Memetic Mutation: The fruit of David's efforts to rescue people? People cosplaying as superheroes and taking pictures of themselves, which now goes past the hardcore geeks and to everyday people. Plus, a whole bunch of them actually start training to be superheroes like Kick-Ass, Big-Daddy and Hit-Girl.
Also, "Tunk," the new swear word that Dave's friends made up (the male equivalent of Cunt) catches on unbelievably fast.
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 Kick-Ass 2 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.
The Scrappy: Big Daddy for turning his daughter into a killer just because he was bored with his life.
Ship Tease: For better or for worse, a lot of the scenes between Kick-Ass and Hit-Girl are framed from an odd angle where it looks like one is kissing the other (the dialogue bubbles dispel the notion, but the imagery is there), in addition to the two being about as close as anyone in the comic can be. The very large age difference may put it into No Yay territory, but Hit-Girl being both more mature and more experienced may not, so it's a YMMV/perception thing.
After Kick-Ass gets hit by the car, if you look at his legs, you can see part of the bone sticking out.
Kick-Ass shoots John Genovese in the tunk. You get to see the end of his dick fall to the floor.
Stoic Woobie: Hit Girl. However, not so much after her father died.
Wangst: Dave's narration can be accused of this. But everyone else has their moments too.
The Woobie: Dave and Mindy. Angie Genovese becomes this in Volume 3 where she becomes a social pariah for the crimes Chris did.
What an Idiot: 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?
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.
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.
Actually, there was a brutal logic to Frank ordering Kick-Ass' death. As he explained to Chris, no one knew who Big Daddy was, while everyone knew Kick-Ass. Hence, killing Kick-Ass alongside BD would send a far more powerful message to any would-be costumed vigilantes. Still doesn't take away the fact that it was also For the Evulz.
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.
Wangst: Same reason as the comics, as mentioned above.