My problem with Medium is that it lends this amazing aura of credibility to everything that is published on it. I think they've hit on the design equivalent of the brown note (of South Park fame) which makes readers mentally incontinent vis-a-vis the credibility of the source of the actual text... Or maybe it's just me?
Not just you. Before I found out what Medium was, I thought it was some new journalism franchise with a lot of amateur contributors. (I guess in a weird way that's what blogging is?)
Then again, I only interacted with the site by following links on sites like this, reading the linked item, then closing the tab. My entire impression came from the quality of those posts (took time to see bad ones make it to the front page of HN, for example) and the post page design. If I'd ever clicked around the site or visited the homepage I might have figured it out faster.
These days I expect a level of quality slightly better than Tumblr when I see the medium.com domain name in a link, and am occasionally pleasantly surprised. The image/marketing only went so far.