Highlights

• Enhanced theme control
  • Customize theme options before activating a new theme using Theme Customizer
  • Use Theme Previewer to customize current theme without changing the front-end design

• Custom Headers
  • Improved Custom Headers with flexible sizes
  • Selecting Custom Header Images and Custom Background Images from Media Library Screen

• Media improvements
  • Support HTML in image captions

• Under the Hood improvements
  • Improvements in WordPress internationalization and localization (more info)
  • Different split in translation POT files for faster translations
  • Codex XML-RPC information update accessed via XML-RPC_WordPress_API
  • WP_Query improvements

http://codex.wordpress.org/Version_3.4