Earlier quoted context omitted.
Can you share an image inline? Can you rapidly search the history? Can you customize alerts? Does it work reliably on mobile? You're debugging something and you want to share a screenshot, inline, how does that work on IRC? Can you start audio or video calls with room participants? Slack is great. I don't need to waste time writing bots because all of the tools I use already have simple integrations: GitHub, Box, Dro…
I think most of the "Why not IRC?" sentiment comes from a viewpoint of "I only use chat for sending short, ASCII sentences." The parent comment really captures the additional use-cases which make Slack much more productive for developers discussing code.
Most IRC clients support unicode. Also, I've never had an issue just using an internal pastebin to share code-snippets with other devs. If we needed to work on code together, we would use an application like remote desktop or a shared screen session.