Audio/Video API
The new audio/video APIs give developers access to powerful media metadata, like ID3 tags.
Semantic Markup
Themes can now choose improved HTML5 markup for comment forms, search forms, and comment lists.
JavaScript Utilities
Handy JavaScript utilities ease common tasks like Ajax requests, templating, and Backbone view management.
Shortcode Improvements
Search content for shortcodes with has_shortcode() and adjust shortcode attributes with a new filter.
Revision Control
Fine-grained revision controls allow you to keep a different number of revisions for each post type.
External Libraries
New and updated libraries: MediaElement.js, jQuery 1.10.2, jQuery UI 1.10.3, jQuery Migrate, Backbone 1.0.