Putting on my designer hat I wrote a post reviewing ReachJS (link).
The one thing I sadly pulled out because of narrative, was PropTypes. In my experience with Angular, Knockout and other front end tools I learned that being able to type check variables being passed in is essential. Most of my code would check the type as it gets passed into a directive or a function and console log out any discrepancies. React not only has this feature built it, it promotes it as a valuable part of the process.