While I'm more concerned about the desensitization to sex (see below), I think that anything "bad" needs to be taken in the context of the people consuming it. If kids are consuming violent content without parental supervision, then they have no one to look to when they see something violent.
I watched plenty of violent stuff as a kid -- pro wrestling before it was widely "accepted" to be fake, for example, and the end of Star Trek II was pretty bloody. But I never turned out to be a violent person.
When GTA and games like it are used as babysitters, that's when you see the problems.
I'm also concerned that "watching out for violence to protect our children" is the rallying cry for most legislators trying to take away the freedoms of intelligent citizens to enjoy what they want to enjoy as long as it's not causing direct harm to anyone else.
For example, legislating sex. Nice dovetail there, no?
I think that, more than violence, I'm concerned about desensitization to sex. Not that I want to see less of it -- I *heart* sex
-- but the mystery of sex has been lost. As a kid, I thought sex was pretty damn mysterious (the "having sex", not the biology; school taught me that part). If I was 11 or 12 now, I would already know what sex was and might even have had it.
There's just something about the mystery of sex that is alluring to me, and I'm sad that today's kids won't have that same experience when they're old enough.