Welcome message
The dashboard home screen displays a welcome message after a new WordPress installation.
Co-Editing notifications
If more than one person is editing the same document, everyone is notified.
Tumblr importer
Using Tools -> Import, data from Tumblr can be imported.
Widgets
Widget configuration for every single theme is stored. If you change between themes, you no longer have to change them from their default positions.
Permalinks
Permalinks are more customizable.
Post slugs
Some characters in post titles would previously cause weird post slugs. This has been fixed.
jQuery and jQuery UI
WordPress now updated to the latest version of jQuery, 1.7.1. It also includes the entire jQuery UI stack.
is_main_query()
Using this method, we can findout if the object is the main WordPress query or a secondary query.
WP_Screen API
Working with admin screens is easier with this new API.
Editor API
The new editor API now pulls in JS and CSS.
Flyout menus
The dashboard navigation makes use of flyout menus, collapsing and expanding as needed.
Admin + Header = Toolbar
The admin bar and the header has been combined into one toolbar. These toolbar makes use of submenus.
Responsive design
The dashboard screen has been updated using elements of responsive design to fit and support a wide array of devices.
Help
Help has gotten a complete update and has been categorised and broken down into smaller topics.
New feature pointers
Every new feature added is now explained by pointers.
Update changelog
Changes made in the WordPress update can be viewed by clicking the W logo in the corner of the toolbar.
Easier uploading
Uploading has been made easier than before.
Filetype detection
Filetypes are now automatically detected while updated.
Drag & Drop media uploader
Uploading is easy as dragging and dropping into the media uploader.
New file formats supported in uploading
Rar and 7z file formats to the list of allowed file types in the uploader.