The animation that happens when Expose is activated, the "Genie Effect" for minimizing windows, and the reflections of dock icons that show up when the dock is at the bottom of the screen are examples that come immediately to mind. (Arstechnica enumerated some of the UI regressions in their Leopard review.)
The genie effect tells you where in the dock the window is, and the expose effect tells you which desktop you're currently on.
You'd think that there would be a standard place for minimized windows to go so that it isn't needed or something...
I feel Expose is quite counter-productive.
The golden rule any developer must know: never use the mouse. Multiple desktops and Quicksilver (instead of the dock, which is quite distracting) are far more efficient.
You can use Expose with your arrow keys instead of the mouse.
In Leopard they introduced reflections and the blue dots to the dock... Both eye candy over the old rev in my view...