IRC is not even remotely comparable. It doesn't save messages, you can't upload media, you can't delete or edit messages, you can't do most things that slack does. Why would you even bring that up.
Does Slack add more features? Sure, and some of them are pretty great. But is it a revolution over IRC to the point that you sully your mouth by uttering them in the same sentence? Not at all. At the end of the day they are both room-oriented text based communication channels.
That said, most horrible part about Slack is how more and more closed it is becoming; when there was XMPP bridge you could at least use some non-shitty client (hello, Electron) to interact with it.
- Having media in chats is obnoxious; links can be passed along in IRC too (and data, although DCC sucks).
- Editing/Deletion of messages actually makes the threads later on harder to understand as someone may have commented on the original message.