INDEXED SEARCH
The indexed search extension has been extended with template support. Additionally to the traditional one, a table-less template is shipped with the package.

The result browser has been rewritten, crawler support was improved, and speed improvements were implemented. Finally, the extension is now able to index Oasis OpenDocument files.

EXTENSION MANAGER
The extension manager has been overhauled and works with the new TER 2.0 now. A new dependency handling is in place, as well as support for the new translation scheme introduced (see also Installation and Upgrade below).

Dynamic image creation / GIFBUILDER
The GIFBUILDER TypoScript objects can create true colour images for PNG and JPEG.

NEW DEFAULT SKIN FOR THE BE
A fresh installation of TYPO3 4.0 will use the new default skin "t3skin" for the Backend (system extension). When you update your existing installation to TYPO3 4.0 and you want to use the new skin, you can install it via the extension manager. Its also possible to use the old skin in a fresh installation. Just uninstall the extension "t3skin" via the extension manager.

UNDO and  HISTORY
The undo and history feature has been enhanced. A rollback and preview on page and record level is possible now. The history now includes inserts and deletes. You can also see the history of an record via the log module.

TYPOSCRIPT
The TypoScript object CONTENT now has a new ".slide" property by which you can fetch Content Elements from parent pages for inheritance, so they will show on subpages. This works similar to TypoScript's getText leveltitle, leveluid feature. 

TypoScript conditions may be linked with OR and AND. 
 
The new := assignement for TypoScript allows now parse time manipulation of TS values (eg "obj.val := appendString(foobar)", "obj.list := addToList(3,5,9)"). 

TSConfig
Is's now possible to override every Page TSConfig via User TSConfig. 

ERROR AND SYSTEM LOGGING
A system log can be used to write errors to a file or syslog (eg DB errors). The target is configured via $CONF_VARS['SYS']['systemLog'] in the install tool. 

USABILITY IMPROVEMENTS IN THE BACK END

Pages can be moved by drag and drop within the page tree in the Backend. The same is possible for folders in the filelist module.

The context menus in the Backend opens much faster now (implemented with AJAX).

Fixed fonts are used in the Backend configuration (i.e. TypoScript setup field or TSConfig fields) for increased readability.

Tab indenting in Backend configuration fields (i.e. TypoScript setup field or TSConfig fields) for enhanced layout is now possible.

The content element wizard got some improvements. Empty categories (i.e. for Plugins) are removed if a Backend user has no right to create them.

The RTE wizard has now a save-and-close button.

User admin module: You can switch back to admin user after user switch (SU-mode). It is now possible to create an new user in the user admin module.

Files can be created or uploaded with buttons in the Web>List content frame.

The admin panel uses HTTP GET instead of POST (no more browser warnings when reloading a previewed page).
Security

New option for lockSSL: get redirected to standard http after BE login with SSL (configurable via install tool)
Accessibility

TYPO3's output is now valid XHTML (the conforming DTD is configurable)

The overall DTD and the XHTML-DTD may be set independently (TypoScript config.docytpe and config.xhtmlDoctype). For more details please see [4].

Output of CSS styled content is now accessible by default (mailforms, menus, sitemaps, Images, Text with Images, etc.).

Images have accessibility features. You can set an "alt" and a "title" attribute for  every image. (integration of extension "cron_cssstyledimgtext")

The "table" content element has now limited accessibility support (integration of extension "accessible_tables")

[by Thomas Esders, www.typo3.org ]