The new XNU voodoo 9.6 kernel - so that everything stays compatible, we need a new kernel for 10.5.6. XNU dev's are currently busy with other projects like new Chameleon bootloader, which delays the next 9.6 kernel.
Chameleon bootloader - new features like GUI, simplified dual boot methods, supposedly more compatible.
10.5.6 - this update should fix some security issues and the overall stability of OSX. Many users reported the opposite. Without the new kernel, the whole system can be unstable. People will have to replace .kexts to older 10.5.5 versions, to get their hardware to work. Not mounting drives, lost of sleep/hibernation functionality, USB not working are the most common problems. Many users decided to downgrade.