http://en.wikipedia.org/wiki/Wikipedia:Replies_to_common_objectionsTrolls and flamers
{Objection:}
"Wikipedia is going to end up like Usenet — just a bunch of flame wars."
{Reply:}
This is a bit more of a problem, but it is dealt with fairly handily by the social mores of Wikipedia, aka Wikiquette. Arguments on article pages get moved either to a corresponding talk page (e.g. talk:theory of relativity) or to a new article page which presents the arguments within a neutral context (e.g., operating system advocacy).
The argument on the talk pages tends to be centered on how to improve the article, rather than on the merits of various competing views. We have an informal but widely-respected policy against using talk pages for partisan wrangling that has nothing to do with improving articles.
Usenet lacks at least two features that are absolutely essential to Wikipedia's success: (1) on Usenet, you can't edit other people's work, while we can here on Wikipedia, thereby encouraging creative and collegial collaboration; or more strongly, on Wikipedia, there's no such thing as "other people's work", because there's no ownership of information; (2) Unlike Wikipedia, Usenet does not have the possibility of enforcing community-agreed standards. Moreover, Usenet is a debate forum. Wikipedia is, very self-consciously, an encyclopedia project! This provides at least some agreement on What Wikipedia is not.
The Wiki way is to focus on agreement, not disagreement as weblog or mailing list or Usenet often do. It is fair to say that there is room for almost anyone to work on Wikipedia, without necessarily encountering those who have a truly incompatible view.
At any given time, there are probably a few trolls and flamers trying to stir up trouble on Wikipedia. But while these folks can make a lot of noise, the great majority of work on Wikipedia continues without paying much attention to them.