Flickr's group discussions use a classic three-level structure, group / topic / reply. This structure goes back literally decades, to Picospan in 1983 and even Plato Notesfiles in 1973. It facilitates conversation without overwhelming people. It's a keeper. However, Flickr's current UI for the discussions is, well, meh. This is too bad. For many people, group discussions are the heart of Flickr. For them it's not so much a photo sharing site as a social network where most of the users happen to be photographers. They deserve a discussion system that is excellent, not meh.