I installed a WordPress plugin last night that adds several buttons to the comment form, allowing commentors to automatically drop in the appropriate HTML for bold, italics, blockquotes, etc.
Simply select the text you wish to modify and click the appropriate quicktag button. HTML tags will be added before and after the selected text.
If you click a quicktag without first selecting text, the opening tag will be inserted at the cursor. Simply add your text, then click the quicktag again to close the tag.