I ended up reading this after reading that:
". . .the plan outlined in #810 simply wouldn't work. The "constructive" nature of a particular comment is far too subjective, even for a person, to judge another poster by their appraisal of it. If you're only letting comments get posted that have received that sort of approval, then you're essentially letting the hoards of bots be your moderating team, an awful idea if there ever was one. (And as a one-time forum owner, I can promise you that bots are FAR more numerous than legitimate posters).
So, we have three possible outcomes:
1) Bots rate all comments as constructive. Posts continue through as though unmoderated, and significantly more spam makes it through than a captcha would allow.
2) Bots rate all comments as not constructive. The board grinds to a halt as all comments, regardless of their quality, are discarded before making it to the message board.
3) Bots rate roughly half of the comments as constructive, and half as not constructive. The rating system fails, as all comments receive roughly the same rating (the hundreds of bots outweigh the few real users and render their ratings essentially meaningless). Comments are either posted or blocked as a fluke, and enough spam gets through to make it worthwhile. Again, a captcha would be more effective."