There are a few threads on here about poor performance on animation effects in Leopard. Some users, myself included, have reported that Spaces, Stacks, and even the Genie Effect all appear somewhat "choppy" or at least not as smooth as in Tiger. The "choppiness" can be observed most clearly when one holds down the shift key before triggering an animation (e.g., before clicking on a stack). Give it a try, then vote. Are the animations "choppier" in Leopard than in Tiger?