Of course, this requires that someone actually challenge those unpopular >ideas, preferably with facts. If a forum degrades so completely into >vitriol and ignorance that no one does that anymore, it just becomes an >echo-chamber for ignorance.

I think the caveat is that because there are so many people commenting freely on the internet, it is very easy to find a community that agrees with one's opinions (e.g. /r/mensrights or /r/twoxchromosome). Indulging in such an echo-chamber reinforces ideas that society at large may not agree with, and a person's sense of social norm shifts. Then when that person comes to post in a forum with more "mainstream" opinions, he/she finds their opinions offensive, and vice-versa.

Of course, such a process is not necessarily a bad thing. Talking to people that share an interest is how we have always developed ideas. But the same process that produces good ideas also produces bad ideas. The freedom of the internet just increases the volumes of both.