key and ref moved off props object, now accessible on the element directly
React is now BSD licensed with accompanying Patents grant
Default prop resolution has moved to Element creation time instead of mount time, making them effectively static
React.__internals is removed - it was exposed for DevTools which no longer needs access
Composite Component functions can no longer be called directly - they must be wrapped with React.createFactory first. This is handled for you when using JSX.

React.renderComponent --> React.render
React.renderComponentToString --> React.renderToString
React.renderComponentToStaticMarkup --> React.renderToStaticMarkup
React.isValidComponent --> React.isValidElement
React.PropTypes.component --> React.PropTypes.element
React.PropTypes.renderable --> React.PropTypes.node
DEPRECATED React.isValidClass
DEPRECATED instance.transferPropsTo
DEPRECATED Returning false from event handlers to preventDefault
DEPRECATED Convenience Constructor usage as function, instead wrap with React.createFactory
DEPRECATED use of key={null} to assign implicit keys

Better handling of events and updates in nested results, fixing value restoration in "layered" controlled components
Correctly treat event.getModifierState as case sensitive
Improved normalization of event.charCode
Better error stacks when involving autobound methods
Removed DevTools message when the DevTools are installed
Correctly detect required language features across browsers
Fixed support for some HTML attributes:
- list updates correctly now
- scrollLeft, scrollTop removed, these should not be specified as props
Improved error messages

React.addons.update uses assign instead of copyProperties which does hasOwnProperty checks. Properties on prototypes will no longer be updated correctly.

Enforced convention: lower case tag names are always treated as HTML tags, upper case tag names are always treated as composite components
JSX no longer transforms to simple function calls