I like the idea of a WP API. I guess in the future you could more easily build non-browser apps that utilize WP data. Of course, there's always going to be the people who don't very much like the whole front-end JavaScript MVC idea (Backbone, React, Angular, etc.), but there will still be the regular PHP themes for them to use.

My only wish is that a proliferation of horrible themes based on inefficient JS does not occur. Front-end MVC can be done right, but don't abuse it with bad code that gives the rest a bad name!