Disqus, one of the best comment systems, added the one thing it was missing: subscribing to comments via email. That means that when you post a comment on a blog powered by Disqus (such as Web Upd8), you have the option to subscribe to that comment thread and you will receive an email whenever someone replies to that post.
The "Subscribe by email" option appears in 2 places: once, at the top of the comments thread and under your name and email form, when you comment.
We started using Disqus primarily for it's speed compared to it's alternatives, but in the mean time, it has truly become great, one of it's best features being the social media comments which we love here at Web Upd8.