My feeling is that mass media and commercial entertainment exert a pervasive and negative effect on our culture. I could cite any number of examples of media that I would view as destructive, but let me first define my terms.
When I discuss popular culture and media, I am thinking of television, cinema, popular music, etc -- entertainment designed for consumption by large audiences by commercial sponsors.
It seems to me that media of this type target our basest instincts and negatively reinforce our development. I think that people who consume large amounts of media become less thoughtful and self-reflective over time, make poorer choices, and are less likely to develop virtues of character. I also think media numb our natural empathy and compassion for others, and encourage us to seek affirmation and success in destructive ways.
By keeping us constantly distracted, I also think that media dampen our creative and innovative output. I think that society as a whole is made poorer when our fellow citizens' minds and creative talents are tied up by the consumption of entertainment.