Good question. My department just switched to Slack for our chats and I really don't see what the big deal is. It's great, but I don't see why it stands out.
And the answer is "yes" you're missing a lot.
I like being able to use any client and not a web browser. Doing things like pulling in images/videos is cool, but most of our use of IRC is just communication & "logging" to a degree--bots reporting new deployments, for instance.