The reason those subs are some of the best is not just because they are strongly curated, but also because they are fairly curated based on well defined, sensible rules.

The unaccountable, power-abusing moderators that are being discussed in this thread remove posts and ban users completely arbitrarily. If these mods had a rule system like r/AskHistorians and applied them as fairly, there wouldn't be nearly as much concern over the issue.

Those are very good subreddits (/r/nosleep is another strongly curated, excellent one with a different purpose) but I don't think that can stand up as a general rule at all. /r/NoStupidQuestions, /r/CrazyIdeas, /r/LearnPython...