As someone who almost lived on IRC in the 90s and now employs several people who use Slack almost all day, my take:

- it's an integrated system with a reliable client on iOS, Android, Mac, and PC that all work somewhat similarly and support all of the features

- dragging and dropping media just works

- the way URLs are expanded is nice, I've not seen any IRC client do this as well as Slack

- 'reactions' on chat items

- the search is great and it's integrated with every client on all those aforementioned platforms

- you can set quite complex sets of notifications for specific channels or conversations.. such as don't mobile notify me between certain hours, but then never mobile notify me for our #random junk channel.. stuff like that. Employees are very thankful for this feature.

- custom emojis, we use these more than you'd think.

- having our backend systems ping messages onto Slack is very easy, basically a single liner - I'd probably have to write some sort of HTTP to IRC bridge to replicate this.

- syncing of read/unread messages across clients

It all adds up.