(1) Lots of strategic downvoting. That's why no thread stays above 75%. It happens to comments as well as threads. Some of it is spammers trying to improve their own ranking, and some is probably just trolls.
(2) "Stalker" downvoting, which is when a person retaliates to a comment by downvoting that person's contributions en masse.
I wonder if they've put any machine learning muscle behind this. Is it easy to fix, or is it just a hard problem?
In theory it may be a good idea, but in practice it's essentially a "CENSOR THIS POST" button that people use to get rid of content they don't agree with.
I've also noticed that negative points on a post completely destroys credibility despite cited evidence or a well thought out argument. I've seen identical comments on similar posts in the past where the first few voters determined the post's fate in that if the first few voters voted it down, the post would receive flames and even more downvotes, but if it was initially upvoted, more people would upvote it and it would receive well thought out replies.
Probably the most horrifying part of the downvote is that all it takes is 51% of voters to dislike your post to destroy it. For example, all it takes is 51% of viewers on the politics subreddit to turn the the /r/politics frontpage into a pseudo fox news where every single post praises democrats and slanders republicans.
It strikes me as disgusting that some people actually consider reddit to be a good place to have a controversial discussion when it's so pathetically easy to censor opinions that you don't agree with.
That having been said, I mostly read the programming section of reddit; I'm prepared to believe standards might be lower in e.g. the politics sections.
There are also some small safeguards in place against (2). If you literally go to someone's user page, and start downvoting every post, reddit will start ignoring your votes towards that user in actual score calculations.
This is why downvots (or upvotes) are not registered if you vote from the user-page.
As to the ops questions: you should look into creating a list of subreddits that cover these things you're interested in.