Release Notes: The new Theme Customizer allows theme developers to easily add customization for header images, background images and colors, and other elements with a live preview. Improvements to the XML-RPC API provide access to Custom Post Types and other data. Changes to the WP_Query object improve performance and memory use. Numerous improvements were made in the i18n and l10n systems. Twitter is now a supported oEmbed provider. Touch support in the administrative UI is provided by the jQuery UI Touch Punch library.
Release Notes: A few of this release are global undo and "trash", image editing (crop, rotate, scale, flip), post thumbnail support for themes, batch plugin update and compatibility checking, easy video embedding via the oEmbed standard, the use of rel=canonical for SEO, commentmeta for extending comments, improved API support for custom post types, extensible registration and user profiles, and many other new features, enhancements, and performance improvements.