> The interface, features, mobile app, and simplicity
3 of those 4 things I would class as "simple setup/use".
I can imagine someone saying "we switched to/chose Slack/HipChat/etc over IRC because it's much simpler to administer and a lot more user-friendly for people to communicate with the team".
I cannot imagine someone saying "we switched to/chose Slack/HipChat/etc over IRC because I need my daily dose of cat reaction gifs"
My team uses slack because, alongside simplicity, the gif and emoji integration makes for a fun break from the grind. So, childish as it is, we love it.
The key thing Slack offers most 'customers' (even those who don't pay) compared to something like IRC is the simple setup/use.
I say this as someone who hates Slack, but has had to endure it on a number of client projects.