Political Correctness differs from politeness in its basic relationship to reality.
When one is polite, one refrains from saying or doing anything that is harmful to another person. There is no problem assessing the reality of the situation but there is also the appreciation that calling attention to the issue is unnecessary and can be painful to the person involved. Everyone can agree that rudeness makes daily life more difficult and can lead to unfortunate outcomes. Since it is cost-free to be polite, I highly recommend it in all interpersonal interactions.
Political Correctness, on the other hand, requires a denial and disavowal of unpleasant aspects of reality. When it becomes the accepted norm, it actively distorts the perception of reality and prevents people from recognizing problems and taking appropriate actions.
Today, in a brief sally through the blogosphere, I found a host of articles which directly or indirectly exhibit their genesis in a politically correct world view. The distortion and unreality involved is apparent. My brief summary of a small portion of the blogosphere is in service to my interest in furthering an understanding of the roots of political correctness and the necessity of confronting it at every turn.