Feature: The new and improved Automator.
How it works: When Tiger was released Automator was a highly touted new feature, and though it's certainly developed a certain fan base on sites like Automator.us and MacScripters, I can't help but feel like it hasn't quite not taken off the way Apple had hoped. Leopard brings a substantial upgrade to Automator, most marked by the addition of UI Recording and Playback. Basically, you can have Automator watch you perform a task such as a mouse-click and then save that task as an Automator action that can be integrated into a Workflow. The new Automator also adds a significant degree of sophistication to Workflows by allowing the use of variables.
Who will use it: As I suggested above, Automator is intended for all users, but it hasn't quite lived up to its promise of allowing everyone to "program" their Macs. Nonetheless, the Leopard version may just be the ticket for getting average users to take more control of their Macs.