What bias does--in science, in media, in any situation where information is gathered--is affect what questions you ask.
McArdle suggests that you tend to be skeptical of findings that go against your point of view, but you are more likely to accept at face value findings that confirm your point of view. In other words, journalists are subject to confirmation bias.
I think that the law of asymmetric insight suggests a more serious concern about bias. That law implies that you are likely to under-estimate your own bias considerably. Asymmetric insight means that you think that you understand the other side better than you really do, so that you think you are less biased than you really are.
If most journalists lean in one particular direction, then this seems to me to be more of a problem if the law of asymmetric insight holds than if it does not. That is, if journalists were aware of their biases, they might report in an unbiased manner. But if journalists are convinced that they are unbiased, when in fact they are not, then they will make no attempt to correct for bias. As I see it, the law of asymmetric insight suggests that it is highly unlikely that journalists will report stories objectively, in part because they will not even notice when they are failing to be objective. The only way to get balanced journalism would be to balance the political biases among journalists.
In short, the law of asymmetric insight leads me to be more concerned than I would be otherwise about media bias.