John Gruber is confused:

I still don’t get what Medium is. These new features certainly look pretty, but they make me more confused than ever regarding what Medium, as a whole, is.
Just read Ev Williams’ blurb at the bottom of his Medium page:
I make systems that encourage typing and thinking (Blogger, Twitter, Medium).
Medium is a blogging platform in my eyes (Blogger ‘2.0’). Yeah, I know their goal is to redefine what a magazine is, and I think becoming blog-like is part of it. There’s also a lot of semantical things happening. People saying the same thing with different words.