David Pierce posted a fantastic piece on WIRED today shedding light on the behind-the-scenes Apple Watch development process, including great insight from Apple's VP of Technology Kevin Lynch and human interface group head Alan Dye.
There are a number of interesting tidbits in the article including this about how Apple first experimented with a chronological user interface, not too dissimilar to Pebble Time's Timeline UI, before scrapping it:
As the testing went on, it became evident that the key to making the Watch work was speed. An interaction could last only five seconds, 10 at most. They simplified some features and took others out entirely because they just couldn’t be done quickly enough. Lynch and team had to reengineer the Watch’s software twice before it was sufficiently fast. An early version of the software served you information in a timeline, flowing chronologically from top to bottom. That idea never made it off campus; the ideas that will ship on April 24 are focused on streamlining the time it takes a user to figure out whether something is worth paying attention to.
There's also a bunch of images showing off Apple Watch's complications, San Fransisco font design, and animations. This really caught my eye though, described as a "unique certificate of authenticity" shown when pairing your Watch to your iPhone. Looks a lot more interesting (and a lot less scannable) than a QR code: