Post Formats: Post Formats now have their own UI, and theme authors have access to templating functions to access the structured data.
Twenty Thirteen: We’re shipping this year’s default theme in our first release of the year. Twenty Thirteen is an opinionated, color-rich, blog-centric theme that makes full use of the new Post Formats support.
Audio/Video: You can embed audio and video files into your posts without relying on a plugin or a third party media hosting service.
Autosave: Posts are now autosaved locally. If your browser crashes, your computer dies, or the server goes offline as you’re saving, you won’t lose the your post.
Post Locking: See when someone is currently editing a post, and kick them out of it if they fall asleep at the keyboard.
Nav Menus: Nav menus have been simplified with an accordion-based UI, and a separate tab for bulk-assigning menus to locations.
Revisions: The all-new revisions UI features avatars, a slider that “scrubs” through history, and two-slider range comparisons.