In terms of downvotes, what really irks me and what I often see is people posting factually correct information, but still being sent into faded oblivion because some sect of the community's worldview doesn't agree with the facts.
If someone uses factually correct information to make a comment thread worse, I can see how downvotes could be justified.
Since comment scores was removed this is the only way to signal this to others besides the original commenter.
That said it should not be overused. If it annoys someone I guess they should downvote it but I don't think there is a need to reflexively downvote every time someone adds a friendly meta comment.
(And if people start gaming it for karma farming I guess it should be downvoted relentlessly until that stops :-)
So no, it's probably not as useful as the comment it was referring too, but it was useful (to some of us) as it pertains to the community as a whole.
I'm being sincere here since I also appreciate book recommendations and I get probably half my book recommendations from HN.
Well, in opposing it I especially read the faded comments and upvote any of those that are not completely abhorrent.
Take that, ycombinator.
This seems to happen a lot more frequently here than anywhere else.
I'm not really sure what that says, other than people still read comments that are faded. Also that people shouldn't worry about self-censoring.
I don't have a problem with downvotes or the karma needed to do it, but
I do sometimes wish it were possible to reply to a dead comment, especially if you vouch for it and it's still dead.
Sometimes they're worth defending, or is relevant in a non-obvious way, and sometimes the comment itself is discussion worthy, as it relates to the topic, even if it's wrong or seems trollish.