* no built-in file uploads
* a bot is required for archiving
* a bot is required for notifications
* no native scrollback/history - IRC bouncers solve this problem, but it's not a plausible solution for most non-techies
* the last two issues also mean that mobile solutions are going to be lacking.
It's possible to have IRC as fully featured as slack, but it does mean either rolling your own solution, or using a lot of other software to accommodate everything and everyone.