Now there's a loooong list of this, but after a morning of "Will the Burning Crusade ruin our children and youths" I would love to see this project:
Find a group of 30 year olds. Random selection method of their choice. Interview them about their main interests between the age of 12-18. That will be from 1989 - 1995. What did they use the most time on? How much time did they use on their homework? How were their grades in this period?
Look at their position in society today, how are they doing?
The different types of media have been around for long enough that we can know if youth and children are ruined by their media consumption. Why not check from this end, rather than speculate from the other end?
And yes, lots of methodological traps here. Got to be a very good sociologist to pull this one off well.