From Joe's blog post:

Major Highlights

• Added jQuery support to the core platform.  jQuery will now be distributed as part of the DotNetNuke installation and will be available for use by module developers.
• Added support for Internet Explorer 8 Web Slices.  Administrators can configure any module to use IE8 Web Slices including the ability to set time-to-live and expiration values.
• Removed distinction between admin modules and pages and normal pages.  This allows administrators to easily delegate access to any portion of the application to any group of users.
• Updated the installation services to support manifest files for all extension types.  Now skins, containers, providers and modules are all first class citizens that can be installed and uninstalled.
• Expanded XHTML, WCAG and ADA compliance.
• Refactored core to improve support for Unit Testing.  Refactored several core classes to use interfaces and added a simple component factory to provide dependency injection support. 
• Added ability to deny permissions in the permissions grid.  This new feature extends the permission framework to give administrators greater flexibility in defining permissions.
• Added Widget framework.  The new Widget framework allows you to quickly add JavaScript/html widgets to your site with very little effort.  The framework supports the use of a simple object tag based representation which means you don’t have to know JavaScript in order to add the widgets.
• Added new Object notation for using skin objects in Skins.  Skin designers will no longer need to include separate XML files when creating and packaging skins.  No more funky “[SKINOBJECT]” tags littering your html.  This significantly simplifies the process of creating skins and further opens up skin development to a broader group of designers.  If you include a simple JavaScript reference in your HTML skin, you can even get a full WYSIWYG experience when designing your skin.

Hope that helps,

The core team tried to avoid breaking changes, most DNN3 and DNN4 modules should continue to work as expected.