The (at the time of this writing) new Mac OS X 10.5 Leopard is now installed on my MacBook for a week. Time enough to find the way through a few of the new features. Time Machine is of course an easy way for backing up the system.
Anyway, to me Spaces (virtual desktops) are a lot more important, since I missed this feature before. Of course there was VirtueDesktops (which even featured desktop switching via the motion sensors). However with Spaces you can assign an application to a certain Space.
Terminal now comes with tabs instead of many windows, which also helps keeping the desktops clean. And last but not least Mac OS X now also includes Subversion, Ruby and of course Ruby on Rails, including – but not limited to – gems and Rake.
Only issue: The background image of the login screen needs to be replaced. In case you also like another picture, LoginWindow Manager might be useful for you.