Unlike some, I am not at all surprised that the JSON API has seen slow adoption, and not for the reasons stated thus far.

WordPress has grown like a weed since Matt forked B2, and a necessary-but-not-sufficient reason for that has been that WordPress has being so easy to build with. Designers with no prior programming experience have filled the world with themes and others with little to no prior programming experience have filled the world with plugins, and most of those themes and plugins are useful and effective on small-scale sites.

But working with a REST API and Javascript is not little league and requires more developer skill and experience, both on the client and on the server. And most of the people who are actively building WordPress sites do not have that level of skill and experience (yet?)

Further, among professional developers, people who develop for WordPress are after looked down on, as lesser capable developers. Go to a local PHP or JavaScript meetup and introduce yourself as a WordPress developer and watch the subtle eye rolls by most people there. Or worse, go to a Ruby, Python, Java or .NET meetup and tell them you develop using PHP and then feel the pity ooze. WordPress developers are considered the epsilons of the PHP/Javascript developer world, and PHP developers are considered the epsilon's of the broader developer world. So we are the lowest rung of the caste system.

How does that apply tangibly? The hourly rates and salaries that a WordPress developer can charge are one of the lowest when you compare to other platforms. So those who are agnostic about their platform often go where they make more money.

I don't mean to be negative, but those who ignore reality are blind to to the eventual outcomes (any potential solutions.)

I do not mean to say there are no good WordPress developers. There are some really truly amazing developers who have chosen to focus on WordPress, but people of that calibre make up a tiny percentage of the people who work with WordPress professionally.

And I am not putting the rest of those people down, they are typically highly skilled in other valuable areas such as design or marketing. But they are just in class with even average developers when you consider all development platforms.

To a certain extent I blame this state on the (lack of) guidance from the core team towards meeting the needs of professional developers. But that's only a small bit of it.

Mostly WordPress does not attract top developer talent on a broad scale simply because WordPress is sp easy for someone with very little skill to build something useful. And that's a large part of why WordPress has succeeded where it has. But it also limits it from attracting better developers en masse.

Bringing it back to the REST API and lack of broad adoption: simply put it's going to be a very slow slog because most people who work with WordPress professionally are going to struggle with building things that use it. And I currently don't see that there is much likely to change that.

P.S. I also think that the "Learn JavaScript Deeply" mantra is going to result in some of the people with aptitude for development realizing they can make a lot more money moving to Node.js and Angular/Ember/React, or Meteor, but most others will struggle to achieve the same level that people have been able to achieve with WordPress and PHP.

And that will mean the adoption for REST API and Javascript will be much slower than Matt would prefer.

JMTCW. FWIW.