So, after hearing the commentary, I've pulled a list of scenes that were deleted for the extended cut :
1) During the sequence that shows Kick-Ass getting famous, the camera pans around the various characters as they're watching TV with all of the media coverage on Kick-Ass. There was an added bit with Dave and his dad watching TV on the couch, at which point Dave's dad gives him the taser in order to protect himself (in response to the mugging). I always liked this part in the script because it set up the taser for when it's later used at Rasul's. It also gives a little more screentime with Dave's dad, which is cool. Vaughn also mentions briefly that they cut out a scene of Dave hallucinating in the hospital and "having sex with freaky spiders," but they a) couldn't afford it, and b) wanted to keep the pace going. As such, I don't know if that would end up in the extended cut since it sounds like certain special effects were never done. I know I actually would've liked to see a LITTLE bit of Dave in the hospital, just to see his recuperation process a little bit. But we'll see.
2) Mindy/Hit-Girl and Damon/Big Daddy have a few more scenes. One looks like they're doing reconnaissance outside during the day. Damon is reading a newspaper on a bench while Mindy is doing some research close by. The most detailed look at a deleted scene is one that shows up in the documentary where they actually show part of the behind-the-scenes footage of it being shot (so we even get to hear some dialogue). During this scene, Hit Girl and Big Daddy appear to be on a rooftop as Big Daddy looks through his sniper rifle. Hit Girl mentions that she wants to just go ahead and do D'Amico's place now, at which point Big Daddy lectures her on why that wouldn't be a good idea. In the making-of book, there's also a reference to a scene where Big Daddy first gives Hit Girl her utility belt, but that didn't show up in the extras.
3) Although the details are unclear, it sounds like there is much more attention payed to the unraveling of D'Amico's character in the later part of the film. He does more and more drugs and gets more and more paranoid toward the climax of the film, and essentially starts to go a little crazy.
4) Again, there weren't many details given, but apparently Gigante's character is a bit more fleshed out in the extended cut. Vaughn also mentions that Ginger goon's moments were almost entirely cut due to the various cuts in D'Amico's storyline. I didn't even know who Ginger goon was until Vaughn pointed him out, but apparently he was originally a little more prominent.
5) At the end of the Big Daddy warehouse shootout, in the release version of the film Chris/Red Mist explains to his dad that "Kick-Ass said we're not the only superheroes out there. Maybe he knows who this guy is." Vaughn explains that this close-up was actually a re-shoot that was done in order to cover for footage they took out, and as such, won't be there in the extended cut (since the brief explanation will no longer be necessary).
6) When Dave is looking at his mask at the cemetery, Vaughn mentions that it was a callback to an earlier scene that they cut, establishing that Dave would hang out by his mom's grave and read comic books.
7) Occasionally, the references are very minor, but still intriguing. For instance, when Damon tells Mindy "time to make Frank D'Amico go bye bye" and she throws the star at D'Amicos' portrait, apparently that originally dissolved into a close-up of D'Amico and another scene with him (much like the earlier scene when Damon is painting the picture of D'Amico). What that scene actually is remains to be scene.
8) After Big Daddy dies, there was a scene of Kick-Ass and Hit Girl getting into the car that Vaughn decided was too comical and jarring for the tone of the film at that point.
9) The Orgy with RM/KA.