And packaging. Just picking on one of the ones you've suggested: Quassel is an app, so I can't easily try it out. I certainly can't invite everyone in my company to use it by clicking a single link in an e-mail. It doesn't have an iPhone app, I think those are quite popular. And so on.
Seriously, all these things are pretty easy, as evidenced by the fact they've been working since the 80's in the form of IRC.
The value appears to be in packaging them up as a modern web app (and having a lot of success in tech-industry PR which is where Slack has excelled beyond e.g. HipChat. But even that is partly down to the good packaging of the onboarding process).
Interestingly, that's exactly what I personally am working on: mobile clients for Quassel, easy deployment (single docket container for everything), quassel-as-a-service for people to just invite others, etc.
A web app also exists, but the discoveravility is an issue.
Packing the things up properly, marketing them well, etc is easier, as we also have the advantage of being open source, which helps in open source communities.
Seriously, all these things are pretty easy, as evidenced by the fact they've been working since the 80's in the form of IRC.
The value appears to be in packaging them up as a modern web app (and having a lot of success in tech-industry PR which is where Slack has excelled beyond e.g. HipChat. But even that is partly down to the good packaging of the onboarding process).