I can't say that I'm a fan of Slack, but in a sense, what they did to IRC (and associated tools like logging bouncers) is what Dropbox did to rsync. Take something that works for people willing to give it time and effort, centralize it, make it a bit flashier and minimize setup to make it accessible to more people.
The user experience for both these tools haven't been great for me, but streamlining them for casual computer users evidently has a lot of commercial value.
This is the point. People keep saying "it is just IRC" but the thing they forget is it is IRC+ with practically no setup. If you want Enterprise or consumer adoption, you must make it easy. Just like what you said with Dropbox. Install client, data automatically available everywhere I log in. That is the product and it actually solves a problem people have.
However, as we have already seen Dropbox has already been commoditized and is looking for a reboot. I see the same thing happening with Slack. It will be the king for a while but it has no moat.
The user experience for both these tools haven't been great for me, but streamlining them for casual computer users evidently has a lot of commercial value.