In a lot of ways, I think Slack's biggest advantage has just been ease of use in modern times: you don't need to know anything about downloading a client and configuring it -- even the friendliest IRC clients I've found are still pretty fiddly compared to "enter your team name, your email address, and your password, boom, done." But this seems like it'd be a relatively easy problem to solve.
I suspect downvotes are from people who aren't comfortable with the idea of what's new... Is actually old and improving.
It's kind of laid out like some irc clients, have you used an irc client before?
Besides taking my comment literally (I now have clarified), lets look at it from a higher level:
The concept of channels, usernames, private messaging in Slack is almost all directly inherited from IRC. There is some great new functionality on top of this core, IRC inspired experience.
It doesn't preclude from the issues of slack being solved by another group.
edit: For the IRC uninitiated who enjoy drive-by downvoting: Slack is IRC-inspired - the concept of channels, usernames, private messaging and bots.... in slack is almost all directly inherited from IRC.