I like knowing that it's going to handle unicode properly, in all cases, for everyone on the Slack channel. I like that it handles right-to-left text for everyone reading.
I like not having to ever think about, is my bouncer down? I also like not having to think about, where am I going to run my bouncer? It's nice not having to think about that stuff at all. I've got a lot on my plate, keeping a bouncer up and running somewhere (just so I don't lose history!) doesn't need to be another thing I do.
I like the plugins, the integrations with various other services. I like the convenience and polish. No IRC client compares to Slack for integrations and polish. It takes friction out of my daily chat experience.
I've even moved some private irc channel chats to Slack channels, because it's just a more pleasant experience in 2017. No regrets about any of that.