Politeness is an important social value; sadly, however, it has long been in decline, to the detriment of society as a whole. This decline is particularly visible on Internet forums, on many of which incivility rules.
Politeness does not mean meek agreement with whatever anyone else says; one can disagree with others, even forcefully, and still be polite in doing so. One can be assertive without being abusive. But it excludes using swearing, insults, ad hominems, interrupting people, etc.
Politeness is important to the maintenance of rationality; when politeness is dispensed with, rationality invariably declines. Rationality requires that we be willing to give those of differing views a fair hearing, and politeness helps ensure that we do so; when we shout down our opponents with insults, we are being neither polite nor rational. Politeness helps prevent excessive emotion in discussions, and excessive emotion quickly reduces a rational discussion into an irrational one.
Atheist forums and websites in particular are noted for their impoliteness, where crass parodies of religious belief are often preferred to rational arguments. Many atheists will defend this, on the grounds that (in their view) politeness is not morally required — which is unsurprising, in that their worldview leaves them without any objective moral foundations.
Politeness can become problematic online because the Internet is able to bring together people from different backgrounds and societies. Since politeness involves adhering to an often unwritten code which can vary from place to place, or between different groups of people in the same place, web users are at risk of seeming impolite to others, even when they have not transgressed from their own point of view.