Earlier quoted context omitted.
A few months ago slack's early plan was posted here. From day one this was the goal - they knew it was just a chat app so they had to make it feel like something different. Its one of the best marketing coups in recent history and I'd love to see more analysis on how it was pulled off.
I was involved in the early days of IRC, and personally, I'm amazed at how great Slack is. I parachuted into an org which had a not-great email culture and took up Slack as a replacement. It was amazingly great.
At my company, we used to use IRC for internal communication and have since transitioned to Slack. The primary difference I see is that in chat, we see reactions to certain messages, in-line paste snippets, and links that use a slack redirect service.
I don't really see much utility in reactions. In-line paste snippets could just as well be a link I could click on to display the image or document in an external application I launch. And the slack redirect service makes it much harder to search my browser history for the document I was looking for.
I've also noticed that in the browser I use (Firefox), slack will, after a few days of using it in the same tab, become very laggy and non-responsive. This requires me to close the tab and open a new one to free up whatever resources were causing it to slow down so much. This was never a problem in the local application I used for my IRC client.
I'm not very impressed with Slack.