I think the biggest misconception is that video games are marketed towards minors. While some definitely are, I can't think of any example of one that didn't have an appropriate ERSB rating system, like EC, E, OR E+10. To prove this point, show the add campaign for a game and compare it to that for a movie with an equivalent rating. For example, the trailer for F.E.A.R 3 with let's say the American version of the Grudege, or Little Planet with that movie with the cats and dogs that talked. The trailers for both mediums really don't differ that much. Thus how can it be said one is marketed towards children when the other is not, especailly since most video game consumers are adults. Or better yet, explain the rating system, then show a few trailers for variously rated video games (without identifying their rating) then make people assign what they believe to be the appropraite ERSB rating. It's really easy to guess which games would be E and which would be M because the ERSB rating system follows common sense. Now I know, people can counteract this arguement with " Children emulate adults, therefore adult content is always more intriguing to children then content made specifically for them" To which you could use various arguments that have already been stated like but movies, t.v etc.... Or point out how none violent wii games are murdering their more violent XBOX and playstation counterparts (the Wii fit, 22.61 million compared to 8 million for Halo 3 on XBOX 360). This shows how pervasive non violent video games are (to expell the misconception it's an overtly violent medium).
Of course all this doesn't directly apply to the topic of video games increasing agressive behavior, but there points I'm surprised I haven't seen more people make. Plus the evidence to suggest video games increase long term aggression is weak. Just point out that for every 1 school shooter that may have played video games there are millions who do that have never committed a violent crime.