If anyone has turned on "Spaces" in Leopard, you'll know why Apple decided to ship it turned off; the window switcher (Command-Tab) doesn't understand Spaces, and will automatically warp you to the first space that has that application running on it. In practice, this is horrible if you have apps running on different spaces (browsers, terminal, textmate, etc). Luckily, Apple has a way of changing that behavior! In 10.5.2 they introduced a default option to disable space switching on Command-Tab. Open up Terminal or iTerm and type the following:
defaults write com.apple.Dock workspaces-auto-swoosh -bool NO
killall Doc

This will make Command-Tab only switch between applications running in the same Space. However, if you switch to an Application which is not in your current Space, it will still warp you to the Space where it is not warp you between spaces; you much always switch spaces manually. Regardless, it's much better and makes Spaces much more usable. Though, Workspaces in Gnome still feel much better.