As far as forums go, I find that HackerNews has one of the most civil and thought provoking discussions on the internet. This is most likely due to responsible moderation where differences of opinion are not only accepted, but respected. In fact, this forum practically moderates itself as I've seen many times a post gets downvoted into oblivion for making personal attacks or ad-hominem arguments. Perhaps this has to do with our (HackerNews users) disposition as engineers, researchers, and entrepreneurs. I may be blowing smoke here, but my perceptions has been that to be successful in those professions, one needs to accept failures/imperfect solutions and admit their mistakes more than others. For example, when I graduated from uni and entered my first developer gig, I completely wiped out a production database that hadn't had a backup run in over 2 years. I almost cried because I thought I was going to blow a great opportunity my first week on the job. After the wave of emotion finally washed over me, I realized that other departments had the data. It was just up to me to analyze the business processes, figure out where we fit into the flow, and work backwards to restore all the data. Most of us have been humbled by a superior dev at one point or another in our career and if we look back at code we wrote two years ago we realize just how much there is to learn. In addition, I've found that many brilliant engineers have impostor syndrome and there are far fewer in STEM fields that succumb to the Dunning-Kruger effect.
On the other hand, as Reddit has grown in popularity, the tyranny of the majority has reared its intolerant head. It truly is a hive mind. All the posts at the top are agreeing with each other (the dreaded circlejerk) or are a series of puns. Arguments are considered fallacious if there is a spelling or grammatical error, not ever considering that English may not be the posters first language. And when Reddit gets a hard-on for justice, it is so certain of its findings that it accuses innocent people of terrorism. (http://www.businessinsider.com/reddit-falsely-accuses-sunil-...). I don't know if this is due to its surge in popularity and/or that it became the site of choice for those who outgrew 4chan.
There was a TED talk (https://www.ted.com/talks/eli_pariser_beware_online_filter_b...) that warned of the dangers of tailoring services to match our personal tastes. I hoped that the internet would expose people to ways of thinking that were just not available in their own localized social milieu. Instead, online filter bubbles/echo chambers promotes even more speculations, rumors, and lies even though there has never been an era where it's been easy, if one were so inclined to seek the truth. It's just sad that many of the social media sites encourage this behavior.