2) Big Daddy's back story turns out to not be his real back story. At the end of the comics, he reveals that he's really just an accountant that got bored and didn't want his daughter to live a boring life so he left her mom made up the whole thing about her mom getting murdered, and they start kicking ass. HitGirl goes back to live with her mom in the comics.

3) The whole burning building scene was completely different. In the comics, it's a random building, Red Mist doesn't want to go in, Kick-Ass DOES want to go in, and this is when they save the cat. It really only bothered me that in the movie it's Red Mist who wants to go in and Kick-Ass who doesn't.

4)Other small things that don't really matter that much…like there was never a jet pack, but there was a bad ass flame thrower in its place. And the whole scene where they broadcast the last fight for the world to see…it wasn't broadcast in the comics. Big Daddy doesn't dress like Batman, etc.

and lastly, the ONLY change that REALLY bothered me:

5) You don't KNOW that Red Mist is a “bad guy/poser” until the end of the comics when he turns on everyone. This was kind of an “oh shit” moment in the comics, and they completely left that element out of the movie, as you know Red Mist's plan the whole time. Also, in the comics Red Mist doesn't like Kick-Ass and doesn't try to save him from his dad.