This sounds wonderfully non-useful, but the thought came to mind upon reading your post: There could be an option to flag a post as, "I really think I solved this."
The immediate objection is that it will be overused. However, the comments just on this post indicate that many LWer are unwilling to be outgoing, giving a strong indication that it very much would not be overused. I myself am finding several reasons I would hesitate to select that option, even if I were very much of the opinion that I was posting the kind of definitive reply on which this topic is built. For one, I would expect an excessively critical eye would be applied to my post and I might be downvoted even further than I might be had I posted without the "audacity" to think I had "solved the debate."
While writing this reply, I further abstracted that there are two distinct types of discussion happening in comments: Idle thoughts and attempts at definitive resolution. To further help understand this issue, I will be replying to the comments that appear to me to be of the latter variety.
Re-reading this comment, I realize that I didn't specify why the initial thought I considered worthy of sharing was thought in the first place: I was trying to think of ways to increase time-unbounded discussion. It seems to me that the time-bound nature of discussion here is a primary hurdle to overcoming this issue.
Every now and then, I write an LW comment on some topic and feel that the contents of my comment pretty much settles the issue decisively. Instead, the comment seems to get ignored entirely - it either gets very few votes or none, nobody responds to it, and the discussion generally continues as if it had never been posted.
Similarly, every now and then I see somebody else make a post or comment that they clearly feel is decisive, but which doesn't seem very interesting to me. Either it seems to be saying something obvious, or I don't get its connection to the topic at hand in the first place.
This seems like it would be about inferential distance: either the writer doesn't know the things that make the reader experience the comment as uninteresting, or the reader doesn't know the things that make the writer experience the comment as interesting. So there's inferential silence - a sufficiently long inferential distance that a claim doesn't provoke even objections, just uncomprehending or indifferent silence.
But "explain your reasoning in more detail" doesn't seem like it would help with the issue. For one, we often don't know beforehand when people don't share our assumptions. Also, some of the comments or posts that seem to encounter this kind of a fate are already relatively long. For example, Wei Dai wondered why MIRI-affiliated people don't often respond to his posts that raise criticisms, and I essentially replied that I found the content of his post relatively obvious so didn't have much to say.
Perhaps people could more often explicitly comment if they notice that something that a poster seems to consider a big thing doesn't seem very interesting or meaningful to them, and briefly explain why? Even a sentence or two might be helpful for the original poster.