The killer feature is that lists are shared publicly. You can set them to be private, but we think sharing of lists could take off. An ecosystem of Web sites recommending shared lists could spring up like that which has grown around Spotify's playlist feature, such as ShareMyPlaylists.
Grouping of your Twitter buddies is a key feature of third-party apps such as TweetDeck and Seesmic. Filtering like this means you can follow more people without being swamped by a huge feed. You can also filter tweets by subject. CNET UK can be at the top of your technology-related list, with separate lists for close friends, funniest tweeters, and those with the most cutting insights into Come Dine With Me.
Lists are being tested on the site itself by a select number of users. The feature will be rolled out in the Twitter API shortly, so other apps could potentially add grouping too.