Another release of WordPress (version 2.7) is expected to be released this week. The big development is the addition of threaded comments. Threaded, or nested comments, are a way of displaying comments that allow visitors to reply to a previous viewer's comment as well as your blog post itself. The advantage of using threaded comments (one solution is the popular Disqus platform) is the increased/enhanced sense of community such an offering provides.
It is important to note that if you choose to upgrade to version 2.7 of WordPress, you do not have to add threaded comments support to your theme. So if you are already using Disqus or another threaded comment plugin, you can continue to use it without any problem. The issue with built-in threaded comment support is that many of the themes currently in use do not support threaded comments, so there will be a little tweaking neccessary to get it working properly.