Requesting a reason for a mod via an associated dropdown is not hard for anyone to understand - and I would expect users of "Hacker News" to be able to figure it out even if it were slightly difficult.

The "lightweight feedback" is "throwaway feedback", because you really only see how unpopular a comment is (is it dead yet? oop! there it goes!). Only the owner knows how popular it is, and even then, doesn't know if it's just a bunch of 'yeah, me too!' or 'that was informative or insightful!'

The most egregious example of 'why is this modded so' is this comment, so much so that I've stuck it in my profile:

http://news.ycombinator.com/item?id=3087822

The thread was about RMS and what an eccentric he is, and about his comments subsequent to Steve Jobs' death. The comment is defending RMS using an oft-quoted verse of Jobs', where he defends mavericks making their own rules and violating the status quo. The comment makes a very insightful link, and is not offensive or malicious. Yet it's downmodded to dead status, something that other comments in the same thread didn't have happen despite being much more fractious. Why was it downmodded? Who can say? It's probably fanboys, nothing else makes sense from a mature point of view.

Which reminds me of another issue with the mod system: it's totally anonymous. There is no accountability. You can diminish someone else's statement without giving a reason why nor standing behind it with your own name/pseudonym. Again, the forum I come from gives you the option to see who modded a comment whichever way - which while there is the occasional problem, is overall a significant net gain for an online community that strives to keep things mature.