You probably activated scale mode for the open windows by clicking the icon (with multiple windows open), since the window previews are disabled by default (due to a not-so-hot implementation of the feature).
In fact, that plugin does not work so well with Unity, so you would be better off using a dock.
From your comments, it seems like you did something to mess with the settings that has disabled the scale mode. If you don't mind having to add your favourites to the launcher again, you can run:
Also, by the way, installing a dock is simple. Open the Software Center and search for "dock", and then choose one you like. The are some good ones such as Avant Window Navigator, Cairo Dock, and Docky. All three support showing the window previews, but not for multiple windows.
For that, you would need to install DockBarX, and use it in a standalone way.
sudo add-apt-repository ppa:dockbar-main/ppa
sudo apt-get update
sudo apt-get install dockbarx
sudo apt-get install dockbarx-themes-extra
dockbarx. You can modify the options show that it activates scale mode when there are multiple windows open and you click on it.