Yes, we’ve already updated our site! 😉
The release of WordPress v3.4, dubbed “Green”, includes a few fun new features, and more importantly, some great performance improvements. View the short highlight video from WordPress.org, and read through the list of features and improvements shown after the upgrade, below.
Live Theme Previews
Try on New Themes
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
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.
Performance Improvements Under the Hood
There are hundreds of under-the-hood improvements in this release, notably in the XML-RPC, themes, and custom header APIs, and significant performance improvements in WP_Query and the translation system. The Codex has a pretty good summary of the developer features.
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.
Read all the details on the official blog announcement: http://wordpress.org/news/2012/06/green/