Bloggers love comments. Comment is our weapon of choice to fight amongst ourselves and comment is the way for us to show our love and appreciation for a great blog. On a serious note, comments make a blog post more interesting by adding new viewpoints. They also increase stickiness by building a community feeling amongst the users. Hence you must actively incentivize your users to comment on your posts. These are some of the wordpress plugins that are expected to encourage discussion on your blog.
CommentLuv This plugin shows a link to the latest blog post of the commentator. Definite encouragement for other bloggers to leave a comment on your blog especially if your blog gets decent traffic.
Top Commentators widget Shows the top commentators of your blog. You can select the duration over which the top commentator is calculated. Avoid the temptation to set it to 1 day as then only spammers will show up. If you set the time period to 7 days or a month, regular commentators on your blog get a free link from you.
Show an email to first time commentator: It does exactly what it says i.e sends an email to the first time commentator on your blog. I wish it could check my rss subscription list and send a message asking him to subscribe to RSS as well. (Let me know if you know any such plugin)
Most recent comments Displays the recent comments on your sidebar. Keeps the discussion alive by getting attention to posts where discussion is going on.
subscribe to comments plugin lets a user know when someone has replied to a comment that she has posted. Works like a charm in getting some passionate discussion going.
Nested comments is a great way to keep different threads of discussion among the users separate. Not necessary for smaller blogs with less than 10 comments per post though.
Finally the comment highlighter allows you to make the comments by the author stand out by highlighting the same in a different color.
Let me know your experience with plugins related to comments that you might have used.