WordPress 3.4 is is now available for download or update within your WordPress dashboard.
This release includes significant improvements to theme customization, custom headers, Twitter embeds, and image captions.
The biggest change in 3.4 is the theme customizer which allows you to play around with various looks and settings for your current theme or one you’re thinking about switching to without publishing those changes to the whole world. For themes that support it, you can change colors, backgrounds, and of course custom image headers. The WordPress team have more planned for the customizer down the road.
Throughout the rest of the admin you’ll notice tweaks to make your everyday life easier. For example, if you have lots of themes the new version has made it quicker to browse them all at once without paging. Version 3.4 also made it possible to use images from your media library to populate custom headers, and for you to choose the height and width of your header images.
The WordPress team have expanded the embed support to include tweets: just put a Twitter permalink on its own line in the post editor and it will turn it into a beautiful embedded Tweet. And finally, image captions have been improved to allow HTML, like links, in them.
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, and you can always dive into Trac directly.