Besides the obvious fact that contextual menus are now in inverted type and resemble overlays (making them easier to read), the Dock preferences has a hidden gem.

Minimized windows can now disappear from the dock.

To find the minimized windows, click and hold to activate exposé (note the lightbox background)—the minimized windows now appear as a smaller preview in the lower part of the screen.

Nice. But the UI seems to resemble Ajax’d websites more and more.