I wonder what the reasoning was behind branding the author in the URL path as opposed to as a subdomain. It seems like a very strange approach to take from guys who have done the latter before.
If nothing else it sure makes posts from Medium very annoying on HN because I can't actually tell which blogger's post I'm going to read until I click the link (primarily read on iPad so no hovering).
SEO would be one theory. Pages under the same 'www' subdomain will provide a more positive SEO boost to medium.com than pages under various subdomains.
Why, simply because it's hosted on a different domain? It seems to help people reach an audience wider than they would otherwise, which is anything but bad for branding.
Seriously. The first thing I do if I strongly like or dislike an article is click the name/avatar to get more information about the author to better understand their background. Medium pushes me onto a page with the rest of their articles instead of a profile and I then have to hope that I can use Google to find their Twitter or website based on the small amount of information in their user blurb (because it doesn't force them to include either).