I'd say for 90% of what I do on Slack IRC is very comparable. It has channels so you can organize your chats around certain topics or ad-hoc create one to discuss between only a handful of people. It also has one-to-one chats. Sure, it doesn't have media sharing (unless you count the quite terrible file-sending feature) but it's not terribly often I need to send files over Slack which aren't already stored in some shared storage so I can simply send a link (which also means that access to documents is managed in a single place, generally a good thing). And as for delete/edit, I hardly ever do this. Most of the time people fully well understand me if I mistype something, and there's always the trusty asterisk correction which is faster to type anyways. And one thing IRC does which Slack doesn't is show me who's in the room, which at least gives an indication as to who might have seen my message.

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.