Do you think the good/bad feelings people get from an upvote/downvote are more relative to other people's karma [...], or do you think they're more independent of other people's karma?

I have no one's experience to go on but my own, but here was mine: when I first started commenting on LW, I was nervous and afraid of downvotes and I didn't care at all about other people's karma; I just wanted to not look terribly stupid. Now that I have been here for a long time and made probably many more comments than I should have (for my health), the main things that bother me are when a downvote is unexpected or when I make a comment and a reply is contradicting me and that reply has more karma than the comment I made. And other times when my comments seem to me to be just as good as other comments in the thread, but get nowhere near as much karma. And also, I'm not entirely sure if we want to encourage the type of person who is hypersensitive to downvotes to come here. My gut says we do, but it also says we relax our desire to not have bad comments too much relative to this type of person's actual value. Complex question.

In particular, Harry Potter comments

I know! If total karma were hidden, though, it wouldn't as much of a problem. People could just weight against the inflation in 'fun' threads like that. (Ever notice how fun-type threads inflate karma?) I also agree with you that posts in Main should have 3x karma instead of 10x. (I was pretty shocked when I found out it was 10x after writing this; if it had gotten the same score (though it wouldn't have)...)

Two points where I disagree with you on what to downvote are very short comments

This is one place where you and I just seem to have different preferences. Maybe it's because you have an easier time understanding dense material, but I seem to require more words. As an unfair analogy, compare reading the Twelve Virtues of Rationality to the entire Sequences. Many lessons from the latter are in the former, but it's really hard to get them out of the former without the latter. I made a modest effort at this and actually surprised myself at how much I got right, but most people do not do this.

and what you call "signals of pseudo-modesty"

I have since changed my opinion of this slightly; since underconfidence is a rare enough sin I usually respond to it with a private message with inquiry rather than a downvote (note also that I mentioned this in the context of thinking a comment is looking for karma; I acknowledge how low the base rate is on this[!]).

Edit: Generally, karma has gotten massively inflated lately. Looking at both Top Comments sections, nearly all of the first 600 are from the last five months.