Xgl enables a few features that have been available on X but not supported, like the alpha-channel visuals which are only now being used by applications. These features fall in the "enabler" category. On top of these enabling features you might have seen a number of vute visual effects, candy, but what is most important is that these features are now available for application developers.
X is no longer to be your bitmap-based, raster-operation grand father operating system. It is a modern windowing system that happens to run all of your old applications as well.
Visually, Xgl will also render all the exposed regions of applications. If an application is not responding fast enough (or not responding at all) it does not miss-paint the screen. Applications that do not respond for a while are grayed out. Then there are all the other visual effects that you have learned to appreciate (like shadows, alpha blended menus).
Learning wise, the cube is a virtual-desktop that people can understand because they can visualize where their applications are (as opposed to "why did everything go away?").
And finally, the zoom-anywhere-at-a-click I find very useful.