Like many who have written on this topic before, I would argue that the bias of the mainstream media is generally independent of the left- or right-wing (with the exception of the obvious outlets such as Fox News) and are instead driven by the profit motive. If we were to look at the motivation of the right accusing the media of being liberal (i.e. left-wing), or vice-versa, I would suggest that what both sides are really saying is that neither one is happy unless their perspective is the dominant one. Despite calls from both sides for a free press, I don’t see either the left- or right-wing being very interested in dissenting opinion.
That is not to say, however, that news outlets, for the most part, do not leave out much of the news and discussion worth knowing and listening to. That’s obvious. But once again, I would point that to the short attention span and intellectual laziness of the audience, coupled with the overwhelming obligations to advertisers.