[michaelt]

Slack adds persistence, shared sessions across multiple PCs and phones, search, accounts with logins, messages to offline users, image and file hosting, and works even if you're heavily firewalled.

i.e. the stuff a veteran IRC user cobbles together from an always-on server running irssi+screen, nickserv, saved identify and ghost commands, a chatbot and some personal webspace.

Still, I agree Slack is far more the child of IRC than the child of Wave. IIRC Wave had full rich text, a threading-like mechanic, and you could edit other people's messages, and documents rather than channels.