Allow your commenters to use bold, italic, link and quote tags while posting comment on your WordPress based site.
Install and activate Basic Comment Quicktags. Using this plugin you can add bold, italic, link and quote tags on comment form and also bbPress if you use. It uses WordPress Quicktag API for displaying these buttons. You can turn these on and off for bbPress or comments by visiting your Dashboard -> Settings -> Discussions page as shown in the screenshot.
Note: After activating the plugin you have to turn its features on from the Settings-> Discussion page.