How about a who's online feature. if someone votes, updates a playlist, or makes a post (sift talk or video comment) update the user in the online database. After 10 minutes remove anyone who hasn't been active in the last 10 minutes.

Another interesting feature would be a who's viewing this post (store the last action the user took and show on the video anyone who's viewing that post) kinda like the youtube active sharing.

Actions might be viewing the queue, viewing the main page, or viewing a specific video). Could also have submitting a video, writing a sift-talk post.