Since the last edition of this book was published, quite a staggering number of new features have been added to the WordPress software. If you're new to WordPress, this list may not mean a whole lot to you, but if you're familiar with WordPress and have been using it for a long time, you'll find this list quite enlightening.
Internal linking available through the standard "add link" box
Inclusion of the admin bar when browsing the blog while being logged in
Full-screen mode for editing posts and pages
Custom menus that can be included anywhere in the predefined areas within the current theme
Faster page load times
Faster upgrades
Dropped support for Internet Explorer 6
Inclusion of the single upload button (with file type detection)
Drag-and-drop media uploading
Responsive design of the admin panel (Dashboard)
The possibility to select custom header images and custom background images from the Media Library
Improved internationalization and localization features
Renaming HTML Editor in the edit post/page screen to Text Editor
New media manager makes it easier than ever to manage photos, videos, and other media files through a beautiful user interface
Galleries can be created faster with drag-and-drop reordering and simplified controls
New welcome screen in the Dashboard
High-Dots-Per-Inch (HiDPI) compatible Dashboard design (also known as Retina-ready)
Better accessibility for screen readers, touch devices, and keyboard users
XML-RPC is always enabled by default and supports fetching users, managing post revisions, and searching
All buttons updated to a modern shape (more rectangular)
Autosave and post locking, together with the new revisions functionality for easy content editing
In-line login feature to save expired user sessions
Automatic maintenance and security updates in the background
Automatic installation of language files (localization)
If you'd like to see detailed lists of all new features added since WordPress version 3.0, take a look at these links:
Also, you can read a fully explained feature list at http://wordpress.org/about/features/.