>there's an explicitly far left-wing message board that doesn't ban people for their politics; because all the more moderate (and moderate cosplaying as extreme) left-wing boards do, that board collects various libertarians, nationalists, groypers and whatnot who can't go anywhere else. Makes for an odd mix.
That actually sounds pretty fascinating. What board are you thinking of?
It's a common observation that any free speech place on the internet will disproportionately attract right-wing wackos. But arguably that says more about the left than the right. If your politics are sufficiently to the left, there are a lot of places on the internet that will cater really well to that, and delete/downvote non-conforming views pretty aggressively (thinking of reddit in particular). So arguably the bottleneck on having a true "free speech forum", where all perspectives are represented, is that people with left politics have more fun options, in the form of forums which are moderated aggressively to cater to their views.
I tried posting on TheMotte a few times, but I found it to basically be a right-wing circlejerk. It was actually a bit eye opening -- before that, part of me wondered whether the stifling conformity on reddit was intrinsic to its left politics. If a critical mass of left-wing posters had been present on TheMotte, I might have stuck around.
I think upvoting/downvoting systems really accelerate the tendency towards herd mentality. It becomes very obvious when you hold an unpopular minority view, and that's a strong motivator for people with minority views to leave.