Oh, I'm sure if you can make IRC work, it might be enough. But aside from the fact that this can be a significant hurdle in many situations, Slack's integration

Oh, I'm sure if you can make IRC work, it might be enough. But aside from the fact that this can be a significant hurdle in many situations, Slack's integration with various tools is a significant advantage that IRC can't quite replace (easily). For example, we've linked our various trello boards to their relevant channels, and as a result when I get into work (part-time) I can simply scroll through the 'front-end' channel and not only see the discussions between co-front-enders, but also see the updates on trello tickets in the same place, in the exact context of the conversations around it. That's become quite valuable to me.