For me the kiiller feature is the ease of use. Here at the BBC we (“we” being developers, sysadmins, and other nerds) use IRC to discuss issues and communicate. It’s a pain in the arse to get onto even as a technical person, though – the BBC has an internal proxy for everything on the network so it’s not trivial to connect. I have to remember to run a specific IRC command every time I start a session in order to attach my client to the right server, and that’s before I’ve even joined a channel.
Contrast that to my setting up my (non-technical) team on Slack: took all of five minutes and they almost instantly grabbed the paradigm (even if some of it comes from IRC). I’d be stuck teaching them how to run
/join commands and negotiate the firewall otherwise.
Then there’s the integrations: I have a channel that runs off our GitHub account and notifies everyone when I commit a change. Took about 2 minutes to set up. I’m sure IRC bots exist to do that but again, way harder. My other team members have added their own integrations easily – again, only a developer would be able to do that in IRC land.
TL;DR: there’s no single Shiny Thing to point to, it’s the whole package. It works easily and without requirement of existing (obscure) skills, and looks/feels beautiful.