Switch from using id attribute to data-reactid to track DOM nodes. This allows you to integrate with other JS and CSS libraries more easily.
Support for more DOM elements and attributes (e.g., <canvas>)
Improved server-side rendering APIs. React.renderComponentToString(<component>, callback) allows you to use React on the server and generate markup which can be sent down to the browser.
prop improvements: validation and default values. Read our blog post for details...
Support for the key prop, which allows for finer control over reconciliation. Read the docs for details...
Removed React.autoBind. Read our blog post for details...
Improvements to forms. We've written wrappers around <input>, <textarea>, <option>, and <select> in order to standardize many inconsistencies in browser implementations. This includes support for defaultValue, and improved implementation of the onChange event, and circuit completion. Read the docs for details...
We've implemented an improved synthetic event system that conforms to the W3C spec.
Updates to your component are batched now, which may result in a significantly faster re-render of components. this.setState now takes an optional callback as it's second parameter. If you were using onClick={this.setState.bind(this, state)} previously, you'll want to make sure you add a third parameter so that the event is not treated as the callback.

Support for comment nodes <div>{/* this is a comment and won't be rendered */}</div>
Children are now transformed directly into arguments instead of being wrapped in an array E.g. <div><Component1/><Component2/></div> is transformed into React.DOM.div(null, Component1(null), Component2(null)). Previously this would be transformed into React.DOM.div(null, [Component1(null), Component2(null)]). If you were using React without JSX previously, your code should still work.