I think it'd be helpful to have a small textbox to add a short comment to a poster where I can put "I agree!" or "Fallacious reasoning" or "inappropriate discussion" that only shows up in the poster's view so there is some feedback besides Up/Down, yet doesn't clog up the thread.

I've never seen that function in a forum though, so perhaps the programming is simple.

Me too.

In seriousness, I was thinking that allowing us to see who has upvoted our comments/posts would probably be helpful and encouraging, although hiding who has downvoted would help protect the voter's integrity and help avoid downvotes being taken as a personal insult.

The risk would be the development of identifiable cult followings, undeserved reciprocation of upvotes, and similar.

I have seen other forums that use this mechanism. They list which users "liked" the post right underneath the post itself. Those forums did not have a karma system, though, and it might seem that the systems are somewhat redundant, but I, for one, would process the two types of feedback differently in my meat-brain.

In short, I sign the above comment without reservation.