Vista introduced buttons that “glow” outside of their boundaries with the minimize/maximize/close buttons in the upper-right corner of a window.
Windows 7 adds another button to the glow-club: the “Start Orb.”
Normal Start Orb
Start Orb with mouse-over
Buttons usually have a different look when you mouse-over them so that the user understands they can click on them.
The glow effect is the same, except it spills out of the button’s boundaries. I supposed the spill-over effect makes it more clear that this is a system button instead of an application button?
…or maybe it just looks cool.