I think the most common reason people probably '--' a node is that they don't agree with the tone. I know most of my posts that have gotten '--'ed (or at least those that I've noticed have negitive rep, as I don't regularly use somthing like repwalker, but that's a different node) I've reread and noticed that they sounded disrespectful to either the Monastery as a whole or to the poster. (Which is almost always accidently; sometimes I let anger from my nonperlish life leak into my posts.)