Persistent history is easy. Email notification is easy. Storage of various assets like text snippets and pictures is easy. I've never used any, but I'm assuming that at least one of the web interfaces for IRC works well...
I think the main thing that Slack has done is package it up so that you don't need to cobble together 100 different things -- which is, of course, very valuable. Or at least more valuable than the monthly cost that they charge ;-)
To be honest, I would really rather be using free software. I would be quite happy to pay for a service that made it easy for me (as Slack does), but software freedom is valuable to me.