One example of downmodding that I've seen personally is that someone will post a reference of a link or a book that I had never heard about. When it is particularly valuable to me I like to respond thanking them for posting it. I don't expect karma from this, I simply want to convey my thanks to the person who provided the information and to encourage other people to do the same. I also upvote them, but I think the personal thanks is more valuable. I've had these "that you" comments downvoted consistently, to the point where I no longer thank people and ultimately delete the comment. I find this behavior strange in a "community".