This one caught me out for a while! In Tiger I would drag one or two application alias' to the Finder Toolbar. When I upgraded to Leopard I found they were still in place but I was missing Quick Look. Basically the Quick Look icon didn't install because it did not have the room. Easily fixed then by simply removing the App alias' and then right-click on the toolbar - choose 'customize toolbar' and then drag the default set to the toolbar. There is then still room to drag one or two Apps back up there.