* The ability to directly send files and images to channels is an objective benefit Slack has over IRC, which has even less ability to do that than MIME email (at least MIME lets you post the base64'd file contents of your image, rather than just a clickable link).
* The ability to send long messages or whole source files is an objective benefit Slack has over IRC.
* The ability to, by default and without running a secondary service that can crash or lose connectivity, log and search channels is an objective Benefit Slack has over IRC, which can support that feature only by using bots, and only by tunneling results through privmsgs or forcing users to leave the IRC protocol itself and visit ancillary web sites.
* The ability to edit and delete messages is an objective benefit Slack has over IRC.
There's probably a bunch more. Don't get me wrong: I don't think Slack is amazing; it's good for what it is, but in 2015 it's pretty unremarkable. The issue is that IRC is so. bad.
There are probably still people making fun of poseurs and hipsters who use IRC instead of ICB. The OpenBSD team used to be like that.
Interpreting and displaying those DCC'ed files or cutting longer messages into sub messages and recombining them is something that you could implement via the IRC client just as easily as it was implemented in Slack (not to say that it would be easy, it'd be a lot of work, but it likely would not be much more work than what took to make that function in slack).
If you're modifying IRC anyway, you can modify the server to record logs and make those available to clients.
Same goes for editing.
IRC is not as bad as you think it is, and it wouldn't be an insurmountable effort to make it as good as slack, but open sourced rather than proprietary, which was the entire point of the original article.
Given the problem of sharing a diagram with a team of developers on a group message channel, I'm sure there's someone here who would say they prefer the DCC option of receiving a named file and then opening it with a file viewer, rather than simply dragging the file into the chat window and having it appear instantly to everyone on the channel. The existence of that person is not interesting to me.