We’ve upgraded to the latest and greatest this afternoon. It took about 15 seconds, so your experience should have been pretty seamless. Find out what’s new after the jump…

Live Theme Previews

Gone are the days of rushing to update your header, background, and the like as soon as you activate a new theme. You can now customize these options before activating a new theme. Note: this feature is available for installed themes only.

Customize Current Theme

Satisfy your curiosity and try on a fresh coat of paint — you can also use the live preview mode to customize your current theme. Look for the Customize link on the Themes screen.

Custom Headers

Flexible Sizes

You can decide for yourself how tall or wide your custom header image should be. From now on, themes will provide a recommended image size for custom headers rather than a fixed requirement. Note: this feature requires theme support.
Choose from Media Library

Tired of re-uploading the same custom header image every time you check out a new theme? Now you can choose header images from your media library for easier customization.

Twitter Embeds

Share Tweets with Style

You can now embed individual tweets in posts. It includes action links that allow readers to reply to, retweet, and favorite the tweet without leaving your site. Just paste a tweet URL on its own line.

This works with URLs from some other sites, too. For more, see the Codex article on Embeds.

Better Captions

HTML Support

Basic HTML support has been added to the caption field in the image uploader. This allows you to add links — great for photo credits or licensing details — and basic formatting such as bold and italicized text.

Under the Hood

Faster WP_Query

Post queries have been optimized to improve performance, especially for sites with large databases.

Faster Translations

The number of strings loaded on the front end was greatly reduced, resulting in faster front page load times for localized installations. Also, better support for East Asian languages, right-to-left languages, theme translations, and more.

Themes API

WP_Theme, wp_get_themes(), wp_get_theme(). Faster, uses less memory, makes use of persistent caching.
Custom Header and Background API

Custom header and background API relocated into the theme support API.

XML-RPC API

A new WordPress API that supports custom content types and taxonomies, as well as dozens of other bug fixes and improvements.
External Libraries

jQuery, jQuery UI, TinyMCE, Plupload, PHPMailer, SimplePie, and other libraries were updated. jQuery UI Touch Punch was introduced.