When you leave a comment on a WordPress.com blog, you can choose to automatically receive email updates about other comments on that specific post by checking the Notify me of follow-up comments via email checkbox that appears in the comment form.
Depending on the email client you use, the notifications will look something like this:
Note that you do not need to be registered with WordPress.com to follow comments. Visitors who are logged in to WordPress.com will not need to confirm their request to follow a thread; all other visitors will receive a confirmation email with full instructions on how to activate their follow request.
Enabling Follow Comments on Your Site
To allow users to follow comments on your posts, visit Settings > Discussion and ensure that both Comment Reply Via Email and Follow Comments are enabled:
Managing Post / Comment Notification
If you want to stop receiving comment notification or start receiving notification on all comments on a particular blog, click the “Subscription Options” link at the bottom of the notification email. You can also use the link shown below.
Full details on managing the posts/comment threads that you follow can be found on the Follow feature support page.
Blog owners can see how many people are following comments on their posts from the Site Stats page: