The fact that Scoop doesn't have this is probably an outgrowth of my not wanting it for K5. It appears that no one else has needed it badly enough to implement it yet, either. But I don't like the idea, and I very much doubt it would be used on K5 even if it were available in Scoop.
See, here's the thing. Thresholds are all about personalization. Most of Scoop's design is about collaboration. This seems like a tiny thing, probably, but it really is a conflict of core values. Personalization features seem like a fine idea, but they subtly promote the idea that you are not part of a larger community. That is, if you don't like the community's decision, you can simply overrule it for yourself. Sounds harmless, but when everyone does it, than there no longer is any community.
The same philosophy is why I've managed to turn aside the intermittent calls for an open rejected-stories page for so long. Basically, I think community enforcement of standards absolutely rests on the idea that everyone has to live with what is decided. If enough people say a comment stays visible, it stays visible for everyone. As soon as most people can hide what they personally don't like, that ethos falls apart, and the site becomes a collection of individuals, rather than a true community.
I know it sounds melodramatic, but I truly think a rating threshold would be disastrous. It would lead to more crap, not less, as those who really care, like yourself, simply walled themselves off. Eventually, most of you would get sick of the steadily increasng numbers of comments below your threshold (which stay because the people who care don't see them, and grow because the negative pressure against posting them is gone) and one by one, you'd head off to the next site willing to go through the same mistake-cycle.
This may or may not be an appropriate discussion for scoop.k5, as I'm not at all opposed to including the feature in Scoop, as long as someone wants to write it. But I doubt it will ever appear on K5, which seems to be more what you're concerned about. But hell, I assume people reading this are interested in community social issues, so argue away. :-)