Installed WordPress 3.4 beta 2 on a test server.
- Theme Customizer with Previewer
- Flexible Custom Header Sizes
- Selecting Custom Header and Background Images from Media Library
- Better experience searching for and choosing a theme
- New XML-RPC API for external and mobile applications
- New API for registering theme support for custom headers and backgrounds
- Performance improvements to WP_Query by splitting the query (Please test!)
- Internationalization improvements (improved performance and locale support)
- Performance and API improvements when working with lists of installed themes
- Support for installing child themes from the WordPress Themes Directory