This means that Dashboard now slides in from the left of the screen when activated, and as there is nothing behind it, it has an opaque patterned background. As a result, Apple has decided to disable the fancy ripple effect that used to be shown when adding new widgets.
If you find that you can’t live without the ripple, don’t worry, you can bring it back by unticking just one checkbox. Open up System Preferences, and go to the Mission Control section.
Then simply untick the first checkbox - Show Dashboard as a Space.
This completely restores Dashboard to the way it was in Snow Leopard, including the fancy ripple effect. This also stops the horizontal swipe to activating Dashboard, but you can bring this back using other tools if you really want it.