The comic, for me, comes down to the fact that the characters have no arc, or story, but are there more to make Miller's point. Whereas it's taking more of a Wanted (the comic version) line of that people are fucking sick and love violence. Fine, but redundant. Watchmen already explored the fetishistic aspect of costumed heroes, and the ascended fanboy angle, but with a plot and characters.
Dave getting Katie is wish-fulfilment, but the film had that tone long before then. From the moment he wakes up from the operation with deadened nerves it's a film about ascension to true heroism, culminating with him strapping on the jet-pack and actually, honest to god flying.
It's why the change to Big Daddy works so well. In the comic not only is his character pointless (more on that shortly) but, while his death sucks, he's a lying piece of shit. Hit-Girl and Kick Ass should be pissed, there's no way he (maybe her too) would want to get revenge for him. It is an act fuelled by revenge though. But in the film, although he's still a fruitcake, he was a good man. And Dave got him killed. So stepping up and helping Hit-Girl is an act of genuine heroism. He's wanted to walk away, had the opportunity and doesn't. He takes on someone else's fight for the second time in his life (the first being the glorious beatdown in front of the shop). It's a fulfilment of his wish to be a hero and Katie is his karmic reward.
It's why Katie is softened in the film. There's a slight subtext about real heroism, what it means and to whom. It's why she works in a shitty volunteer centre and has an obsession with "lame-ducks". She's a grounded, more real take on Kick-Ass. Someone who's doing good deeds for a personal reason for fulfilment.
As for Big-Daddy. In the comic he's a twisted mirror of Dave. What he could become if he keeps this shit up and a look at his motivations from another angle. You know who also serves this angle? Red Mist. The film version changes his character just enough to be a perfect fit. He's literally Dave, but with a bad influence and money for toys. He's even less of a hero, wanting to play the part but not managing to beat Dave's paltry two acts of heroism. If there is a sequel Red Mist is going to be a fantastic villain. He's set up to be classic comic book villain. Shared origin, dark reflection, once friends with the heroes and ultimately more powerful. He's going to be dangerous as hell. Unlike the comic version he doesn't aspire to villainy. He want's to be a hero, but also wants his father's approval. Kick-Ass shoves him down the path to villainy, which is a great set up for conflict.
Also wtf did he scream in that part.(not going to spoil it but you know what I'm talking about) Kryp-to-niiiiiite?
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