Memory usage improvements - reduced allocations in core which will help with GC pauses
Performance improvements - in addition to speeding things up, we made some tweaks to stay out of slow path code in V8 and Nitro.
Standardized prop -> DOM attribute process. This previously resulting in additional type checking and overhead as well as confusing cases for users. Now we will always convert your value to a string before inserting it into the DOM.
Support for Selection events.
Support for Composition events.
Support for additional DOM properties (charSet, content, form, httpEquiv, rowSpan, autoCapitalize).
Support for additional SVG properties (rx, ry).
Support for using getInitialState and getDefaultProps in mixins.
Support mounting into iframes.
Bug fixes for controlled form components.
Bug fixes for SVG element creation.
Added React.version.
Added React.isValidClass - Used to determine if a value is a valid component constructor.
Removed React.autoBind - This was deprecated in v0.4 and now properly removed.
Renamed React.unmountAndReleaseReactRootNode to React.unmountComponentAtNode.
Began laying down work for refined performance analysis.
Better support for server-side rendering - react-page has helped improve the stability for server-side rendering.
Made it possible to use React in environments enforcing a strict Content Security Policy. This also makes it possible to use React to build Chrome extensions.

Introduced a separate build with several "addons" which we think can help improve the React experience. We plan to deprecate this in the long-term, instead shipping each as standalone pieces. Read more in the docs.

No longer transform class to className as part of the transform! This is a breaking change - if you were using class, you must change this to className or your components will be visually broken.
Added warnings to the in-browser transformer to make it clear it is not intended for production use.
Improved compatibility for Windows
Improved support for maintaining line numbers when transforming.