I think this is an important consideration. What should be done different to mitigate toxicity?

Honestly it's exciting to be able to ask these sorts of questions and potentially be able to act on it. I'm inclined to say:

1. There's an optimal size to subreddits. Small subreddits can be toxic, large ones are unless they have ultra strict moderation (so basically askhistorians).
2. Massed users from other subreddits can easily overwhelm and torpedo communities, even ones larger than themselves if coordinated enough.
3. There aren't any sort of feedback mechanisms or checks on moderators.