Sign-in with Slack looks cool, and would be easy to support. 

But here's an important question — is storage provided along with the user’s identity? 

What does that mean? Let’s say I have a few JSON files, the user’s data with my app, will you store that for us, or do I have to find another place to put it?

None of the major APIs except for Dropbox, provides storage. If any of the others did it, even with a small amount of data, it would make a huge difference in the kind of apps an individual developer could create.

Think about WordPress-with-Storage, or Facebook-with-Storage, all of a sudden you could do everything in the browser with JavaScript. Why shouldn't a platform like Slack add that basic feature?

I've done it for Twitter, with nodeStorage. It really is lovely, liberating, enabling. Swear to god. This is one of those things like blogging, or RSS -- or Slack -- that once you have it you wonder how you got along without it. 

Update #1: People are confused.  Suppose I'm writing a Markdown editor for WordPress posts. It's going to generate HTML -- that goes into the post, but the source, the Markdown stuff, needs to be stored somewhere. We're talking about the users' documents. Basically every app that works with documents needs a place to store them. That's what I'm talking about. 

Update #2: No storage.