The OSX interface has been fairly consistent since its first release in terms of the general big themes - menu bar at the top, dock at the bottom, unless you choose to move it. What's changed has been the graphical implementation of these elements, as well as the windows and boxes that make up the rest of the applications. The interface improvements in Leopard are a mixture of eye candy tweaks and actual useful changes to the way things work.
High on the list of pointless visual tweaks include the new dock, which now has bevelled edges and extra alpha-reflective-web-2.0-transparency effects, as well as the menu bar, which is now semi-transparent for no particular reason. Apple die-hards will be sad to see the disappearance of the menu bar's rounded corners, which set a rather nice frame for the desktop in previous versions of OSX. Windows now have a consistent visual effect, with the brushed metal of previous versions now pronounced dead.
The new dock allows you to create 'stacks' of icons, and these are basically folders that expand when you click. Stacks, or piles, of documents are a desktop metaphor that Apple has been playing with for almost a decade, but this sees the idea's first implementation. Stacks are useful for creating quick-access folders of your most important files and also for quick-launch groups of applications. However, it's hard to be too enthusiastic about them - they are, after all, just folders with a neat animation.
The new Finder has received much attention for its iTunes-like interface - in fact, Apple advertises it as aping the popular music player's interface. Down the left is now your list of 'sources' - drives, networks and the like - and the main viewing pane now features a Cover Flow view, enabling you to view your folders like album art. In theory this is a great feature with some amazing visuals; in practice, it's pointless for everything bar folders made up entirely of pictures.
Quick Look is the other major new interface adjustment, and it's basically a document viewer that requires minimal opening time. Rather than double-clicking an image in Finder and having it open in Preview, the document viewing application, you can now open the picture in Quick Look, the document-viewing widget. Quick Look also allows you to have a gander at videos and multi-page PDFs from within CoverFlow view. A neat touch, but hardly revolutionary or, for that matter, time-saving.