As a counterpoint: I am in 78 IRC channels. I've never seen slack mentioned anywhere aside from here on hackernews. I suspect it's not actually converting any long-time users, but only being an effective draw on people who like the new and shiny.
Slack has nifty features like integration with tools, and inline gifs, etc. But the really nice thing is that it feels a little more human. Everyone knows that IRC turns people into jerks. Slack won't completely prevent that, but it does help to promote the feeling of community. It's made for teams, so it's kind of designed around it.