From the blog post:

What Group Policy preferences can do for you
Group Policy preference items increase the manageability of Windows Server 2008, Windows Vista, Windows Server 2003, and Windows XP, and reduce the TCO of servers and client computers running those operating systems. With Group Policy preference items, you have:

• Simpler configuration via common user interfaces, and reduced need to create and maintain complex configuration scripts.
• Item-level targeting to give you more precise control over the application of preference settings
• Flexibility for how preference items are applied – a Group Policy administrator can initially configure a preference item, but allow the end user to change the settings later
• Richer configuration within an image, so you need fewer images

• The ability to use Group Policy to manage:

Windows settings
Applications: Extensible for ISVs to create simple Group Policy preference settings for their applications.
Drive Maps: Create, modify, or delete mapped drives, and configure the visibility of all drives.
Environment: Create, modify, or delete environment variables.
Files: Copy, modify the attributes of, replace, or delete files.
Folders: Create, modify, or delete folders.
Ini Files: Add, replace, or delete sections or properties in configuration settings (.ini) or setup information (.inf) files.
Network Shares: Create, modify, or delete ("un-share") shares.
Registry: Copy registry settings and apply them to other computers. Create, replace, or delete registry settings.
Shortcuts: Create, modify, or delete shortcuts.

Control Panel settings
Data Sources: Create, modify, or delete Open Database Connectivity (ODBC) data source names.
Devices: Enable or disable hardware devices or classes of devices.
Folder Options: Configure folder options; create, modify, or delete Open With associations for file name extensions; create, modify, or delete file name extensions associated with a type of files.
Internet Settings: Modify user-configurable Internet settings.
Local Users and Groups: Create, modify, or delete local users and groups.
Network Options: Create, modify, or delete virtual private networking (VPN) or dial-up networking (DUN) connections.
Power Options: Modify power options and create, modify, or delete power schemes.
Printers: Create, modify, or delete TCP/IP, shared, and local printer connections.
Regional Options: Modify regional options.
Scheduled Tasks: Create, modify, or delete scheduled or immediate tasks.
Services: Modify services.
Start Menu: Modify Start menu options.

The full post includes some great links you'll want to pay attention to:
http://blogs.technet.com/grouppolicy/archive/2007/11/16/group-policy-preferences-announced-at-it-forum.aspx