actually, it is more like 1.0.1, but heck! :)
TL;DR: io.js is a fork of node.js.
A few core maintainers of node.js got fed up with the slow updates of node.js, the infinite beta state of 0.11 and that 0.12 was basically coming for two years now, and decided to create their own fork of the server-side JavaScript runtime, called it io.js and released it today.
Read the Q&A for more background!
First, for all the changes, see the CHANGELOG.
Second – this is still in beta, not intended for production. Wait for a more stable release if you want to use it in production (coming hopefully much sooner, than expected from node.js)
Have you heard about ES6? That is, EcmaScript 6, the next version of the JavaScript programming language, bringing a whole host of exciting features! The specification is still currently a draft, but is coming along rather nicely (slated to release/recommendation this year) – you definitely should check it out, because - well - ES6 Rocks!
Anyway, io.js includes support for a whole host of ES6 features – see their blogpost on supported ES6 features! If you want to know more about ES6 features, the 2ality blog is a treasure trove for easy to understand & thorough posts with plenty of examples about a lot of them!
For general ES6 compatibility, refer to Kangax' compatibility tables - or try transpiling to ES5.
Visit http://iojs.org to download, https://github.com/iojs/io.js to contribute, and read the FAQ!