Every Ubuntu release brings new and interesting things onto the Linux table. Karmic Koala is no different. With each new release we see better hardware support, better power management features, better stability and security, and better software in general.
Usability has also been getting much attention recently not just from the Ubuntu developers, but from the open source community in general.
Here's my list of the five best features for Karmic Koala:
New boot experience - From using Grub2 as the default boot loader to using upstart in non-legacy mode, the long term efforts in streamlining Ubuntu's boot process finally gets a big boost as the pieces start falling together. The overall boot process has been optimised in preparation for a 10s boot target in Lucid Lynx and the speed up can already be felt now.
Ubuntu Software Center - Many users (myself included) have long lamented the mess with Linux software installations. Newbies will tend to be confused by the assortment of tools available for installing or removing software on any Linux desktop. The Ubuntu Software Center is the Ubuntu developers' answer. The version in 9.10 is still only the first iteration but it's a good first step in unifying software management on the Ubuntu desktop.
ext4 by default - Newer filesystems are needed to take advantage of new hardware capabilities and ext4 already implements some of these ideas to bring better performance and reliability to the Linux user. I'm using ext4 already for 9.04 and I find it stable enough for regular use.
Improved security - Recent years have seen an increased focus on security, thanks to the malicious software problem plaguing Windows. While Linux has always been more secure by default, that's no reason to be complacent. With each successive Ubuntu release, more and more supported software are being made more secure by default. Existing security features such as AppArmor and uncomplicated firewall have been improved, while Position-Independant Executables (PIE) and NX bit emulation are just some of the new additions to the security stack.
Quickly - No operating system thrives without a healthy developer community. Linux has always had a treasure throve of programming tools for developers that can be quite intimidating to newcomers. Quickly is a unified framework for developers to more easily get into developing software for Linux and Ubuntu, something that the Linux desktop has always needed.
Ubuntu 9.10 Technical Overview