The lifecycle methods componentDidMount and componentDidUpdate no longer receive the root node as a parameter; use this.getDOMNode() instead
Whenever a prop is equal to undefined, the default value returned by getDefaultProps will now be used instead
React.unmountAndReleaseReactRootNode was previously deprecated and has now been removed
React.renderComponentToString is now synchronous and returns the generated HTML string
Full-page rendering (that is, rendering the <html> tag using React) is now supported only when starting with server-rendered markup
On mouse wheel events, deltaY is no longer negated
When prop types validation fails, a warning is logged instead of an error thrown (with the production build of React, type checks are now skipped for performance)
On input, select, and textarea elements, .getValue() is no longer supported; use .getDOMNode().value instead
this.context on components is now reserved for internal use by React

New Features

React now never rethrows errors, so stack traces are more accurate and Chrome's purple break-on-error stop sign now works properly
Added support for SVG tags defs, linearGradient, polygon, radialGradient, stop
Added support for more attributes:
- crossOrigin for CORS requests
- download and hrefLang for <a> tags
- mediaGroup and muted for <audio> and <video> tags
- noValidate and formNoValidate for forms
- property for Open Graph <meta> tags
- sandbox, seamless, and srcDoc for <iframe> tags
- scope for screen readers
- span for <colgroup> tags
Added support for defining propTypes in mixins
Added any, arrayOf, component, oneOfType, renderable, shape to React.PropTypes
Added support for statics on component spec for static component methods
On all events, .currentTarget is now properly set
On keyboard events, .key is now polyfilled in all browsers for special (non-printable) keys
On clipboard events, .clipboardData is now polyfilled in IE
On drag events, .dragTransfer is now present
Added support for onMouseOver and onMouseOut in addition to the existing onMouseEnter and onMouseLeave events
Added support for onLoad and onError on <img> elements
Added support for onReset on <form> elements
The autoFocus attribute is now polyfilled consistently on input, select, and textarea

Bug Fixes

React no longer adds an __owner__ property to each component's props object; passed-in props are now never mutated
When nesting top-level components (e.g., calling React.renderComponent within componentDidMount), events now properly bubble to the parent component
Fixed a case where nesting top-level components would throw an error when updating
Passing an invalid or misspelled propTypes type now throws an error
On mouse enter/leave events, .target, .relatedTarget, and .type are now set properly
On composition events, .data is now properly normalized in IE9 and IE10
CSS property values no longer have px appended for the unitless properties columnCount, flex, flexGrow, flexShrink, lineClamp, order, widows
Fixed a memory leak when unmounting children with a componentWillUnmount handler
Fixed a memory leak when renderComponentToString would store event handlers
Fixed an error that could be thrown when removing form elements during a click handler
Boolean attributes such as disabled are rendered without a value (previously disabled="true", now simply disabled)
key values containing . are now supported
Shortened data-reactid values for performance
Components now always remount when the key property changes
Event handlers are attached to document only when necessary, improving performance in some cases
Events no longer use .returnValue in modern browsers, eliminating a warning in Chrome
scrollLeft and scrollTop are no longer accessed on document.body, eliminating a warning in Chrome
General performance fixes, memory optimizations, improvements to warnings and error messages

React with Addons

React.addons.TestUtils was added to help write unit tests
React.addons.TransitionGroup was renamed to React.addons.CSSTransitionGroup
React.addons.TransitionGroup was added as a more general animation wrapper
React.addons.cloneWithProps was added for cloning components and modifying their props
Bug fix for adding back nodes during an exit transition for CSSTransitionGroup
Bug fix for changing transitionLeave in CSSTransitionGroup
Performance optimizations for CSSTransitionGroup
On checkbox <input> elements, checkedLink is now supported for two-way binding

JSX Compiler and react-tools Package

Whitespace normalization has changed; now space between two tags on the same line will be preserved, while newlines between two tags will be removed
The react-tools npm package no longer includes the React core libraries; use the react package instead.
displayName is now added in more cases, improving error messages and names in the React Dev Tools
Fixed an issue where an invalid token error was thrown after a JSX closing tag
JSXTransformer now uses source maps automatically in modern browsers
JSXTransformer error messages now include the filename and problematic line contents when a file fails to parse