I wish I could find the original document. They do spend about 1/8th talking about how it has to be a great experience. But the majority of the document discusses how they aren't a chat app, they had a broader vision of team communication. And so you don't compare Slack to IRC because IRC is "just" a chat app while Slack is for "collaboration".

In the early days of Slack we had Office Communicator in Microsoft. But because they thought of themselves as essentially MSN messenger for business they didn't do the things Slack did with their clear vision. Ultimately both apps existed to send text/files/images to people in realtime but Slack communicated a vision of how this should work while the rest just let you figure it out yourself.