or maybe it is already automated, we will never know :)

You don't. Arguably, as far as the software is concerned, karma points are a signal for meaning but of course it doesn't work that way in practice. Maybe you could sort by a ratio of upvotes to comments, or recency, obviously leaving flagged threads out, possibly also leaving out threads that can no longer be commented on. But all of that is just sorting by a bad heuristic (karma) with extra steps.

I don't think you should bother trying to find "meaningful" threads at all, since there is no objective definition for that in context, other than "relatedness" to the current thread. I have a browser plugin that does it, it doesn't need to be that deep. Let readers decide what is meaningful to them. You can't automate everything.