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My understanding is that the original developers of node left the project to create io.js, a derivative with a different set of values and goals.

See https://iojs.org/en/faq.html and https://iojs.org/en/es6.html

Check out this episode of JavaScript Jabber for a good explanation of io.js:

http://devchat.tv/js-jabber/147-jsj-io-js-with-isaac-schleut...

tl;dr is that all the core contributors outside Joyent started working on io.js with an open governance model. Everyone wants to join back together but only after Joyent agrees to put node under the same governance model.

It's a 0.12 drop-in replacement with some minor API enhancements and a lot of bug fixes.

Also, all current v8 ES6 features are enabled in io.js by default, while they still require the --harmony flag on node.

Not all of the features, just those marked as stable in v8.