Here is a feature roundup as highlighted in the official About page you will see in the Dashboard.
New Media Manager
Adding media has been streamlined with an all-new experience, making it a breeze to upload files and place them into your posts.
Creating image galleries is faster with drag and drop reordering, inline caption editing, and simplified controls for layout.
New Default Theme
Introducing Twenty Twelve
The newest default theme for WordPress is simple, flexible, and elegant.
What makes it really shine are the design details, like the gorgeous Open Sans typeface and a fully responsive design that looks great on any device.
Naturally, Twenty Twelve supports all the theme features you’ve come to know and love, but it is also designed to be as great for a website as it is for a blog.
So Sharp You Can’t See the Pixels
The WordPress dashboard now looks beautiful on high-resolution screens like those found on the iPad, Kindle Fire HD, Nexus 10, and MacBook Pro with Retina Display. Icons and other visual elements are crystal clear and full of detail.
WordPress supports more usage modes than ever before. Screenreaders, touch devices, and mouseless workflows all have improved ease of use and accessibility.
A number of screens and controls have been refined. For example, a new color picker makes it easier for you to choose that perfect shade of blue.
Under the Hood
Meta Query Additions
WP_User_Query classes now support meta queries just like
WP_Query. Meta queries now support querying for objects without a particular meta key.
Post objects are now instances of a
WP_Postclass, which improves performance by loading selected properties on demand.
Image Editing API
WP_Image_Editor class abstracts image editing functionality such as cropping and scaling, and uses ImageMagick when available.
switch_to_blog() is now significantly faster and more reliable.
The WordPress API is now always enabled, and supports fetching users, editing profiles, managing post revisions, and searching posts.