Medium is a platform born from the minds behind Twitter for publishing and discussing longer-form works. Rather than targeting the rapid back-and-forth microblogging niche Twitter addresses, Medium allows for more detailed contributions, often resembling feature articles or editorials. Quality, user-generated content is rare on the internet; and Medium provides one of the cleanest, least intrusive ways of making it accessible.

Source: Medium
Medium is currently a relatively small service—at least compared to Twitter—but enjoys a strong, growing community of dedicated contributors posting, sharing, recommending, and curating the growing pool of content.
Pinning down an identity for Medium in this nascent stage is tricky. On the one hand it employs many of the extrinsic reward motivations—such as reputation and scoring—popularised by content aggregators and recommender services. I’ve touched on these concepts in previous posts, and Medium compels users to share and rate content in the same ways that Digg, Reddit, and Slashdot have in the past. These services leverage the desires of users by allowing ‘up voting’ of articles to surface—or bury—quality content. This currency of reputation and scoring is paired with a robust system of curation and tagging that tasks users of Medium to thematically organise content and build their own personally-curated collections.
By marrying these two systems together Medium is able to entirely self-organise around user behaviour. It is as meaningful for users to contribute high-quality content as it is for them to gather and collate that content in a collection. Whether they’re authors or curators, users of Medium can be a part of a rich ecosystem where their contributions be recommended, voted, scored, and—most importantly—commented on.
Medium is able to deliver and package this content so successfully because it relies on the same simplicity that propelled Twitter into the stratosphere. It does away with so much of the artifice and bloat associated with most WYSIWYG publishing platforms and offers an elegant, direct platform. Like Twitter, Medium puts the simplicity of content first-and-foremost. It is easy to publish, read, and recommend items because every piece of the workflow is built in service of the user.
Usability and user-centered design is always best when it is simple. Rather than being caught up in the zeitgeist of sacrificing simplicity for the latest interfaces, fads, or ‘feature-added’ hooks Medium is able to provide such a rich user experience by confidently relying on the strength of its contributors and providing as few distractions as possible to the quality of their work.
Simplicity isn’t just a feature for Medium; it is their chief strategic advantage.
Further Reading: I highly recommend Giles Colborne’s book Simple and Usable for any and all of questions you have about user experience and user-centered design.
Colborne, G. (2011). Simple and Usable: Web, Mobile, and Interaction Design. Berkeley: New Riders.