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?
I think they've just managed a good design - mostly by resisting the temptation to show off as so many designers do. It's very minimal - if anything it looks similar to the hand-written no-styles websites that academics often write (that look like http://motherfuckingwebsite.com/ )
:-)