Providing APIs for third-party integration is what Microsoft is almost too good at (hence why Word and Excel are so scarey). The idea that Slack would be so dumb as to believe that Microsoft will do poorly at this is shocking.

Slack is a chat application, one of a vast multitude. There's not all that much secret sauce to give away.

Their secret sauce seems to be in how they can convince so many people to use it despite similar products existing for decades. I really wish I knew the answer to this question, I admire it a lot.

The UX is top notch. Not only is the UI polished, but big details like mentioning someone and an invite link appearing is awesome. Or force-pinging someone on DnD because of an emergency.

Slack is seriously user-centric, all the way down to how they run their Twitter.

Have you used it on an under powered laptop? It's dog slow and gets exponentially worse the longer you have it open. Don't even thinking about scrolling back through the history.

They didn't start with all that, but somehow amassed a lot of users anyway. I believe it's more about the personal networks of the original team, but that's just a guess.

Well, what deep secrets could a chatapp possibly have thats unreplicable.

Can you explain the difference between an open and a closed 3rd party API?

To develop an app using the Slack API, you do not need to register with Slack and download an API key for your app. Anyone can just develop whatever they like without having to ask permission first and there's no easy way to selectively shut down apps that Slack doesn't like.

I guess here the split is between a closed source 3rd party API and no API at all.