If your weblog has a comments form, it is a very good idea to have some kind of small-print on the form that explains how the submitted comment is going to be munged. It's very difficult to comment effectively when I don't know the following:

• How do I start a new paragraph?
• How do I insert a single linebreak?
• How do I make a hyperlink?
• What will happen to any HTML, angle brackets or &entities;?