DNN v3 New Features Summary

First Look

Bolder new default skin - now looks less like SharePoint than in earlier betas
There are new TopPane and BottomPane sections above and below the normal (Left,Content,Right) panes into which you can put content that spans the page
Admin IconBar at top contains shortcuts to commonly used functions. Developers can create custom control panels for Admin functions.
You can drag & drop modules to position them on the page (like SharePoint web parts or ASP.NET v2 web modules). Click in the header bar of a module to begin the drag and drop process.
Admin screens have collapsible sections, helping to organize the admin process
Site search is available through new modules named Search Input and Search Results. Modules must implement the new ISearchable interface to participate in portal search.

Portals

Save / Load template with or without content
Skins & containers developed for v2 will work in v3
Friendly URLs make it possible for search engines to effectively index your pages

Pages

Tabs in v2 are now called Pages in v3
Pages can now point to a URL, another Page on your site, or to a file (leave it set to URL with no Link URL Location value for a normal page with modules and content).
Page management shortcuts on the IconBar: Add, Settings, Delete, Copy, Preview
Copy modules from another page, optionally with content
Page start and end dates for automatic scheduling of content display.
Permissions settings are simplified (and Pages are not publicly visible by default)

Modules

Add new module or point to an existing module on your site, link or copy data
Specify title when adding module in the IconBar to save a configuration step
Drag & drop modules to position them through a new client-side API
Import/export module content to other modules
Module start and end dates for automatic scheduling of content display
Module header and footer let you decorate the module by putting HTML in them
Permissions settings has a new Inherit View Permissions from Page checkbox (but it's not set by default - so you have to check it on each module)

File Manager

New interface makes file management easier and more powerful
Create sub-directories with permissions on roles, allowing you to more easily organize your portal content
Group files for copy/move/delete
Filter files using wildcards (% and _)

Users

New user fields (web, IM, cell, fax)
Preferences section (preferred language, timezone)
Subscriptions public role (under Membership Services at the bottom of your user profile)
Admin search mechanism for users

Language / Localization

Host

Control panel selection (IconBar or classic)
Site log storage can be set to database or file (keeps your database from growing rapidly)
Export portal template makes setting up new portals a snap
Search admin for managing search (re-index)
Lists management (countries, currencies, etc.), and you can create your own lists for your custom modules
You can specify superuser (host) accounts

Portal Application Design (developer items)

Membership/Roles/Profile - the core team has adopted the “Whidbey“ membership provider API (ASP.NET v2). This allows straightforward integration with other apps that have adopted this API, and a relatively easy migration to ASP.NET v2 in the future.
Breaking changes will require all custom modules to be reworked before they'll function on v3. There are many new features that can now be supported, i.e. IPortable (module import/export), ISearchable (portal search), exception handling, logging, localization, etc.
DesktopModules are now separated from the core project
Revamped DNN namespaces with many new features
web.config has changed dramatically with the addtion of many new provider specs
New interfaces that your modules can implement:
- ISearchable for site searching
- IPortable for module export support
- IActionable for custom actions support like menus
- IUpgradeable for running custom actions on module upgrades