The most negative aspect of the media is that it controls what the public perceives and hears about. European and Asian exchange students in my area, for instance, often ask why Americans receive little or no real world news. There is a major network that has an evening program that is supposed to deal with world news, but little other than the mid-East is discussed. The magazine that is named for world news also gives only a select report on select parts of the world.
Major newspapers in the United States are owned by certain people who control the slant of the news, again. One need only look at the treatment of presidents such as Richard Nixon whose conduct was throughout his life constantly under the miscroscope and compare it to the investigations on other congressmen and presidents who have had less than ethical careers themselves. One such example is the senator from and Eastern state who undoubtedly committed murder as a young man, yet he was lauded throughout the media after his death.
The presidential elections are perfect examples of the bias of the media. In fact, Time magazine's editors even admitted to having influence the past election because of their front covers that presented one particular candidate a multitude of times. In another example of news bias, when citizens went to Washington to protest the proposed health care bill, it was reported to the public that a few thousand attended when later it was revealed that actually millions of citizens had gone.
Photographs are good at creating bias, also. Depending upon the angle of the camera, a small group can seem like more people, or many people's picture can be cropped to appear smaller. So often, programs that are decidedly conservative invite prominent guests whose views are conservative, as do liberal programs. Frequently, people of one side or another watch programs that reinforce their beliefs so that they feel justified in what they believe. Regular television shows even affect people's ideas about human relationships, race relationships, and morality. "Sex and the City" and like shows condone behavior heretofore scorned in the America of earlier days.
There are groups that work behind the scenes to expose these biases of the media. Judicial Watch is one such group that prints information withheld by mainstream media. Currently, they are working on an issue that may go before the Supreme Court before it is over.
To read a University of Pennsyvania report on "The Effect of the News Media Environment on Citizen Knowledge of State Politics" see the site listed below.