I've found it virtually impossible to find a sub organized around an intellectual topic or person that isn't virtually unreadable for anyone who knows anything about the topic.

The more specific and concrete (and smaller, above a certain minimum) the sub is, the better the content. Indeed the same - absent the size qualification - could be said of hacker news discussions.

I suspect it may have something to do with the tendency for those with little knowledge of a complex, abstract and highly nuanced subject to upvote or downvote based on some combination of popular bias and sentiment contrasted with the decreased likelihood of those with knowledge to do the same (because they know more, they are less confident in their opinion). Combined with the fact that invariably growth means more users in the former group joining than the latter.

More concrete subjects where comments can be based on specialized experience (rather than just knowledge) help individuals without that experience to exclude themselves from voting and those with to step in. That hn is based around such a concrete topic is what saves it (combined of course with the paid and highly skilled and dedicated moderation).