From the Debian Project site:
This release includes numerous updated software packages, such as the K Desktop Environment 3.5.10 (KDE), an updated version of the GNOME desktop environment 2.22.2, the Xfce 4.4.2 desktop environment, LXDE 0.3.2.1, the GNUstep desktop 7.3, X.Org 7.3, OpenOffice.org 2.4.1, GIMP 2.4.7, Iceweasel 3.0.6 (an unbranded version of Mozilla Firefox), Icedove 126.96.36.199 (an unbranded version of Mozilla Thunderbird), PostgreSQL 8.3.6, MySQL 5.0.51a, GNU Compiler Collection 4.3.2, Linux kernel version 2.6.26, Apache 2.2.9, Samba 3.2.5, Python 2.5.2 and 2.4.6, Perl 5.10.0, PHP 5.2.6, Asterisk 188.8.131.52, Emacs 22, Inkscape 0.46, Nagios 3.06, Xen Hypervisor 3.2.1 (dom0 as well as domU support), OpenJDK 6b11, and more than 23,000 other ready-to-use software packages (built from over 12,000 source packages).
With the integration of X.Org 7.3 the X server autoconfigures itself with most hardware. Newly introduced packages allow the full support of NTFS filesystems and the use of most multimedia keys out of the box. Support for Adobe® Flash® format files is available via the swfdec or Gnash plugins. Overall improvements for notebooks have been introduced, such as out of the box support of CPU frequency scaling. For leisure time several new games have been added, including puzzle games as well as first-person shooters. Also notable is the introduction of “goplay”, a graphical games browser offering filters, search, screenshots and descriptions for games in Debian.
The availability and updates of OpenJDK, GNU Java compiler, GNU Java bytecode interpreter, Classpath and other free versions of Sun’s Java technology, into Debian GNU/Linux 5.0 allow us to ship Java-based applications in Debian’s “main” repository.
Further improvements in system security include the installation of available security updates before the first reboot by the Debian Installer, the reduction of setuid root binaries and open ports in the standard installation, and the use of GCC hardening features in the builds of several security-critical packages. Various applications have specific improvements, too. PHP for example is now built with the Suhosin hardening patch.
For non-native English speaking users the package management systems now support translated package descriptions and will automatically show the description of a package in the native language of the user, if available.
More info can be found here.