WordPress 3.4 dubbed "Green" to honor the guitarist Grant Green is now available, and includes significant improvements to theme customization, custom headers, Twitter embeds, and image captions.
Matt Mullenweg on the dev blog notes, 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."
For users, the biggest change in 3.4 is the theme customizer which allows you to play around with various looks and settings for current theme or one the you're thinking about switching to without publishing those changes to the whole world.
The themes that support this new feature, can let you change colors, backgrounds, and of course custom image headers.
Also, if you have lots of themes, you can now quickly browse them all at once without paging.
Also, made improvements to the admin panel, for example, it's now possible to use images from the media library to populate custom headers, and also added the ability to let you choose the height and width of your header images.
Well, if you use WordPress post editor for publishing, you can include tweets by just putting a Twitter permalink on its own line in the post editor and the software will turn it into a beautiful embedded Tweet.
And, finally, image captions have now been improved, and allow HTML, like links, in them.
Here's a short clip with the highlights Introducing WordPress 3.4 "Green":