Internal Linking – click a button for an internal link and it allows you to search for a post or browse a list of existing content and select it for inclusion.
Admin Bar – contains various links to useful admin screens. By default, the admin bar is displayed when a user is logged in and visiting the site and is not displayed in admin screens for single blog installs. For multisite installs, the admin bar is displayed both when visiting the site and in the admin screens.
Streamlined Writing Interface – new users of WordPress will find the write screen much less cluttered than before, as more of the options are hidden by default. You can click on Screen Options in the top right to bring them back.
Post Formats – meta information that can be used by themes to customize presentation of a post. Read more in the article Post Formats.
Network Admin – move Super Admin menus and related pages out of the regular admin and into a new Network Admin screen.
List-type Admin Screens – sortable columns for list-type screens and better pagination.
Exporter/Importer Overhaul – many under the hood changes including adding author information, better handling for taxonomies and terms, and proper support for navigation menus.
Custom Content Type Improvements – allows developers to generate archive pages, and have better menu and capability controls. Read more in the article Post Types.
Advanced Queries – allows developers to query multiple taxonomies and custom fields.
Refreshed Blue Admin Color Scheme – puts the focus more squarely on your content.