“Sync’d read-state across multiple devices” is what sold me on Slack years ago. I had been trying to simulate it for a long time in IRC using various bouncers, but it never worked very well and was hard to set up for non-technical people at work.
Work had a private irc server that required all sorts of client certificates to join, forget non-technical people -- it wasn't very good for technical people either!
For the record, I don't like Slack very much. I just disagree that IRC or IRCCloud are acceptable alternatives.