WordPress, on their official blog, made a short announcement that Wordpress.com blog owners could now enable reply by email support for comments made to their blog. With a couple of simple configuration changes, blog owners can get the convenience of being able to directly reply to new comments via their preferred email address instead of through the Wordpress admin interface.
Replying to comments by email isn't that new as there are a lot of other comment systems that support it. In fact, there is at least one plugin for stand-alone WordPress blogs that enable this ability as well. But, the key to new features appearing on WordPress.com's free hosting site has always been rigorous usability and load testing first, then a full roll-out of the feature.
The way reply-by-email comment support will work is pretty straightforward. After enabling support for emailing comments, and then turning on the 'Enable sending comment replies via email' option (both found on the Settings > Discussion page), you are set. The next time you get a comment, you will be notified by email, and you can respond simply by replying to the email with your text above the original comment.