Among others, this new release comes with GIMP 2.6, GNOME 2.28, Mozilla Firefox 3.5, OpenOffice.org 3.1, Linux kernel 2.6.31, X.Org 7.5, and the Empathy Instant Messenger instead of Pidgin. The Ubuntu One client, which interfaces with Canonical's new online storage system, is installed by default. It also debuts a new application called the Ubuntu Software Center that unifies package management.
The default filesystem is ext4, and Ubuntu 9.10 has transitioned to Upstart, improving boot performance. On top of that, GRUB 2 is now the default bootloader - however, if you are upgrading a system which still uses the old legacy GRUB, it won't be updated to GRUB 2, as this is a risky operation. The Grub2Testing page explains how to upgrade to GRUB 2 if you wish to do so anyway.
Ubuntu's KDE variant, Kubuntu, also received numerous improvements, such as the new KDE4 Plasma Netbook interface - still in technical preview. It includes KDE 4.3.2, a whole boatload of social networking plasmoids, OpenOffice KDE integration, and a whole lot more.