1. New Desktop - First off is the new desktop, featuring a new menu bar, a snazzed up dock and "Stacks" to help you keep your desktop clean. For instance, there's a default Stack that collects all your downloads in one place on the dock.
2. New Finder - More on the aesthetics side, Apple is going with a unified look for apps, which nixes the brushed metal style and instead mimics the current iTunes theme -- surprise, surprise. In fact, the new Finder looks and performs almost exactly like iTunes, all the way down to integrated Cover Flow for shuffling through your files. You can also save smart searches in the "playlists" side of the interface. On the back end of things, Leopard includes "Back to my Mac," which keeps track of your home Mac's IP address through various (and secure!) magicks, letting you browse your files remotely as if they were on a local network. Spotlight search also works over networks now, as expected.
3. Quick Look - Another new Finder integrated function, Quick Look lets you open up previews of most common document types without opening the respective app, and unsupported doc types can be added through extensions.
Keep reading for the rest!