Not to mention:
1) IRC doesn't support any form of message threading. There's some user conventions like "username:" or "@username" to direct a message to a specific person, but there's no way to make a clear distinction between multiple conversations in a channel. (You can make a new channel for the conversation, I guess, but that's even more hostile than usual to other users trying to follow along.)
2) IRC doesn't support messages longer than ~400-500 bytes. There's an internal hard limit of 512 bytes per line in the IRC protocol, and part of that gets silently consumed by fields like the server hostname and channel name. If you try to send a longer message, it's silently truncated. (Your client will probably echo it back as if it all went through, but other users will see it truncated.) Trying to discuss anything longer than a single line of text typically requires users to post it to an online "pastebin" - adding another external tool you need to make IRC work.
3) There's no reliable, standard way to transfer non-text content on IRC. DCC is unreliable, slow, requires one or both users to set up their firewall to allow incoming connections (!), and only transfers files to one user at a time. All of this combines to make IRC incredibly awkward for discussing audiovisual content, like graphical designs or audio/video. Sure, you can upload screenshots or videos to another web site and post links to IRC - but that's yet another external tool you have to bring in.
4) User authentication and authorization in IRC is incredibly crude. The protocol is built around the understanding that users can change their username at any time (!!), with systems like NickServ bolted on after the fact to "protect" names. User permissions are tied to a user's presence in a channel; private channels are implemented with a single shared password. All of this is "good enough" for a lot of social IRC networks, but completely unsuitable for a business setting which demands a higher level of assurance.
A previous rant of mine on IRC's missing features: https://news.ycombinator.com/item?id=40813743