In terms of a roadmap, we've got pretty big plans—here are some things (including an API/integrations) that people have requested: http://www.milanote.com/poll. But obviously there are lots of things we're planning to build which aren't on that list :)
If I was the Atlassian CEO reading this thread, I'd be really worried. There are a lot of comments from developers with zero faith in Atlassian's ability to create (or even preserve) a great product.
People who say Medium is "just a blogging platform" obviously haven't written many blog posts. Writing a post is easy. The interface to do that is easy. Distribution is hard - that's the bit Medium solves in a revolutionary way: getting people to read your stuff.
I think the fundamental issue is that humans are spatial thinkers - i.e. we think about things in 2 or 3 dimensions, not just one. There's something about being able to see everything mapped out in front of you (with post-its or similar) that just works better.
So while eventually every story becomes linear, the thinking required to get there works best with more dimensions.