To be fair, that same trivial disposed chat concept you mentioned lives in Slack, as well. It's incredibly easy to quickly make a group DM chat from a channel.
> "There's also no expectation that you need to read everything to catch up when you come back after a while - take that to email if you need to."
Slack doesn't require that expectation, either. You can easily change settings of Slack to just ignore messages that you were not there for, just like you are describing.
While I actually do like IRC also, your two points are pretty weak considering Slack does both of those well, too.
> "There's also no expectation that you need to read everything to catch up when you come back after a while - take that to email if you need to."
Slack doesn't require that expectation, either. You can easily change settings of Slack to just ignore messages that you were not there for, just like you are describing.
While I actually do like IRC also, your two points are pretty weak considering Slack does both of those well, too.