My take on the desirable features and uses of a Facebook group:
It should be a closed group (viewers must apply to join and be approved, otherwise it would soon be taken over by spammers) but visible in the groups list, and advertised.
Members should be allowed to post.
Used to bring selected interesting Q/A threads (I guess especially those of a fundamental conceptual nature) to the attention of a wider audience.
Used as a channel for light discussion and chat.