Codex
Version 3.1
On February 23, 2011, WordPress Version 3.1 "Reinhardt", named in honor of the jazz guitarist Django Reinhardt, was released to the public. For more information on this enhancement and bug release, read the WordPress Blog, and see the Changelog for 3.1.
For Version 3.1, the database version (db_version in wp_options) changed to 17056, and the Trac revision was 17485.
Highlights
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.
Implement the screen options toggle sprite across color schemes
Improved admin list type screens, including making most columns sortable, introduction of a new pagination style, improved API, improved search, and filterable bulk actions. (#14579)
Round bottom-right corner of the screen options dropdown
Use 'Visit Site' in header if blog has no name
Add 'Check Again' button and timestamp to update-core. Also return the same error message for themes as we do for plugins when no checkboxes are checked
Editor utilizes the new "formats" option in TinyMCE, fixes errors in IE when pressing Enter inside a caption box, fixes resizing of the caption box when the user soft-resizes the image in the editor
Simplified the post/page edit screens by hiding several meta-boxes--use Screen Options to expose those boxes.