The downside to doing everything over HTTP is that you're completely proprietary until someone else implements your protocol--but they never will, because like with NNTP it's just easier to create their own standard. NNTP still has fairly rich client support within many modern e-mail clients.
Oh, but it did take off. It was the most popular on-line group discussion place twenty years ago. NNTP (or rather Usenet) decline started around the time that web forums popped up in the internets.
Biggest FirstClass features we miss: - Asynchronous (don't need to respond in realtime). - Threads within topics/folders. - Ability to mark item as unread. - Confirmation that message had been read.
That ability to mark item as unread was extremely useful. I'd frequently read something, see it is important, but will take time to do. Simply, mark as unread and go on with life. It is there waiting later in the day (or tomorrow). The message originator knows I've seen the message.
Threads on Slack are clumsy. Tried using them and quickly abandoned them.