Theme preview is now real-time and you don’t need to go back to your dashboard every time you need to make a change to your theme. You can update the header, background, color settings and everything. You can also change the settings before you activate the theme. It helps you make the changes before making the theme live. But this feature is only available for themes which already exists in your theme folder, meaning you can only do this for themes which has already been installed.

The live preview mode can also be made useful to customize your current theme. You don’t need to go to Appearance > Editor to make color changes and other minor changes in appearance. That’s possible in the live-preview section. This feature is good for you, if you play with your site’s appearance a lot. Simply click on the Customize link on the Themes Screen.

You can now specify how much width or height your header image should have. Your themes will provide you with a recommended theme size instead of a fixed requirement for custom headers. This is often useful.

Now you don’t need to re-upload a custom header image, every time you update your site to a new theme. Instead, you can make use of the Media Library to upload a custom header image.  This will help you in customization of themes.

I liked this feature. This is a cool new feature that WordPress implemented in this update. Now, twitter users can have their tweets inside a post or page along with the Retweet, Reply To, and Favorite Button. All you need to do is to, paste your Twitter tweet permalink into your post in a separate line. Tada, you have a mini-twitter on your blog.  Though it’s called Twitter Embeds, this should work for others sites like: 

Now, a basic html support has been added to image captions. Using this feature, you can specify image credits for your image or add basic formatting like bold, italics for your caption text.

If you have’t yet updated to 3.4, be sure to do it. There’s a lot of cool things waiting for you in WP 3.4.