Look at it this way: The book is about the limits of fantasies, about a guy who discovers his dream of being a real superhero is boring until it ends up with him stabbed, tortured, humiliated and in true Peter-Parker-style worse off than when he started. Except that Parker at least knows he helped people while Kick-Ass takes down a drug dealer purely by accident and another superhero with even more elaborate fantasies simply executed with a bullet to the head.
The movie is about how fun it would be to live a superhero's life, about how despite a few setbacks that everybody experiences at the start the hero takes down a drug dealer, gets the girl, makes friends and rides a jaunty jetpack around NYC. In short, the movie wallows in the fantasies that the book attacks.
Admittedly the movie starts off more or less the same but there's one scene where you can see it change. Kick-Ass climbs into Katie's open window at night to admit that he's really Dave, her supposedly gay bff, and oh yeah that he's not gay. Despite initial shock and attacking him with sporting equipment she then decides instead to take him to bed and become his girlfriend. The whole thing is so beyond-implausible that I was sure this was an imaginary sequence, even well after the scene had ended. In the book Dave only reveals that he's not gay, prompting Katie to have him beaten up before she taunts him with her boyfriend and has her other friends ridicule him. A bit extreme but not just more plausible but more to the point. (It may also be worth noting that with this change to Katie and the movie having Mindy's mother actually be dead pretty much every woman in the movie becomes a sex object or a nobody like Red Mist's mom. The book won't win any feminist awards but at least there's some autonomy to the characters.)
It's not that the story is too medium-specific to comics but that Vaughn didn't rethink it enough as a movie. Mostly what he did was change the comics references to movie ones (such as Spider-Man and Batman not the book's She-Hulk and Nova), stretch the story too far then force the ending into something more conventional. It appears that he was originally headed towards the book's ending which is why the scene with Hit-Girl's "origin" clearly states that it's not entirely true. No surprise that nearly all the sex from the book is gone - same thing happened with Wanted. And it's one thing to show an 11-year-old girl as a mass murderer but the book's scene where she sniffs cocaine (believing it to be a power enhancer) clearly had to go. It doesn't help that the movie was made on too-obvious sets and a CGI NYC which only makes it more unrealistic. And I'm still not sure why he revealed Red Mist's identity so early unless it's more of the Hollywood idea that audiences hate surprises. (Which certainly doesn't bode well for the adaptation of Chosen.)
The book isn't exactly subtle and Millar wasn't trying to be but it does have its own consistency and some psychological depth. Millar at least shows Hit-Girl reacting to her father's death - the movie just has her shrug it off like some b-movie action star. It's hard to tell if Chloe Moretz and Nicolas Cage were miscast or if they were deliberately playing up the characters' happy talk falseness - falseness that is a key part of the book but that the movie takes as completely true. But then superhero comics have a fairly long tradition of self-examination that's not common in movies and could even be traced (if you're willing to stretch) back to Don Quixote and Northanger Abbey. Kick-Ass the movie had possibilities but it would have required a filmmaker who actually understood the purpose of the story and was willing to follow that path.