It makes sense at the end of the film that Mindy kisses Dave, especially considering how Dave is basically her only friend around her age, and how it's shown that Dave was there to comfort her when the local Alpha Bitch and her friends pick on her. That, combined with emerging hormones, means it makes perfect sense for Mindy to be attracted to Dave. The movie hints at this first early on when Hit-Girl stares at Dave briefly as he undresses so he can put on the pimp clothing and a little before the climax when Mindy stares at Dave's muscles when he's shirtless.
Dave's father makes him promise to never wear the Kick-Ass outfit again. While in the comics, Dave doesn't respect it, in the movie, he wears Big Daddy's armor with the K-A costume underneath, and in the end of the movie, he works on having a different outfit, more akin to an armor. So, while Dave's father intended for him to stop superheroic activities, Dave doesn't do so, but doesn't break his promise either. That also fits in with the better personality of movie Dave compared to his comic self.
Motherfucker's eventual plan to gather the disenfranchised, bored and psychopathic together and incite them to burn the city to the ground in his name is similar to that of the Nolanverse Joker's efforts to gather a similar group together to burn Gotham to the ground. This draws a direct comparison between Motherfucker and Nolanverse Joker, acting as a deconstruction of the latter. Basically, someone who genuinely thinks burning an entire city to the ground is a good idea is more likely to be an infantile, obnoxious Psychopathic Man Child than a cool, Straw Nihilist badass. Better, Chris quotes the Joker himself at the end of the last movie, and becomes just as psychotic by the end of this one.
The Tumor is the only one of the Motherfucker's goons who doesn't die on-screen. Last time we saw him, his hands were bound and Mindy said she's make him eat his own dick. Now, what did we learn in the scene where Hit-Girl cuts a guy's hand off after he breaks a promise he just made? She Does Not Exaggerate. It helps his chances that he seemingly spilled the beans pretty quickly after hearing that threat.
Go ask the guy in the car crusher if spilling your guts gets you leniency with Hit Girl.