Binary package hint: compiz
Panel shadows are currently enabled by default for anyone running Compiz. While shadows on most windows look quite nice, and can be useful - the shadow underneath the top panel looks ugly when a window is maximised or positioned below the panel. This can look even worse with some themes, and looks worse if the user increases the shadow radius (which can be done easily from ccsm).
See the two screenshots attached - one with the default shadow enabled and one with the shadow disabled.
There is currently no obvious way for a user to disable the shadows on the panels, and I think that they should be disabled by default.
This is easily done by setting gconf key /apps/compiz/plugins/decoration/allscreens/options/shadow_match to '!type=dock|any'
I think it would be a better idea to have the panel "on the same level" as the windows so that windows aren't covered by the panel shadow. OSX does it this way. Check out this screenshot.