No nesting. Is this 2004? How are you supposed to follow a conversation more than two posts deep? This is PHPBB with more javascript.
Nesting (and reordering by votes or activity) works well for divergent discussions: expressing views, exploring the possibility space, brainstorming. Or: exhibition . Flat (and stably ordered by time) offers some hope for a convergence of viewpoints, or at least shared-narrative and common-knowledge. Or: deliberation .
Nesting: discussions converging towards separation, isolation, protectionism and flamewar ;)
Flat: discussions converging towards consent.
EDIT: Getting downvotes, but face the facts: HN, Reddit are pretty exceptional. Look at the legacy from the 90s and try to proof me wrong.