Earlier quoted context omitted.
Same type of questions: what's the difference from other blogging sites? What's the business model? Maybe because of its huge user base? I think all these questions have already been answered though.
Even if medium is never anything more than tumblr with clearer design and restricted to plain text, that will still be a valuable thing.
But that's not the only value there. I guess a lot of people like to post their blogs on Medium which gives a sort professional taste. But I think blog.com and wordpress.com are also very good sites.
I'm just curious about the evaluation from the investors point of view. A service website may not value that much because they don't have a real product. User base may change over night.