I welcome comments that add other perspectives especially those that actively disagree with me in a vigorous way. However, I don't allow negative comments about people. There are plenty of places on the Internet for you to express those opinions.
Another thing I don't like is talking about me in the third person in the comments. Think of a comment here as a Letter to the Editor, and on this blog, the editor is me. If you're talking about me to someone else, either take it offline or skip it.
Update: If a comment is sarcastic or devoid of information or ideas, especially if it is anonymous -- I delete that too. There's no point blocking anonymous people, for obvious reasons. You think someone is talking gibberish but you don't have the guts to put your name on it -- why should anyone care?