A gallery recently posted by Think Secret gives us good insight into the kind of changes Apple is making in Leopard. You should visit the linked page and see the entire gallery for yourself, but the two screenshots above encompass the salient changes.
First of all, most noticeably, Apple is changing the colour of the menu bar to the greyishness you find currently in all the iLife '06 apps. Although I don't find this particularly attractive, particularly because it's become the background colour for the toolbars also, I can see why they might have done it - so that it's easier for the eye to distinguish between the two distinct parts of the window - the content area and the title bar. Because, after all, the title bar is used to move the window around and it's the only part of the window that can do so. Secondly, Apple is simplifying a lot of common applications even further as can be seen by the new super-simplified Software Update application.
Personally, I can't wait for WWDC and to be able to get my hands on a Developers' Beta of Leopard.