Quick Look is a beautiful thing, and in my view practically itself worth the cost of admission to Leopard. Unfortunately, the more you get used to it, the more annoying it is when you get to a file format that Quick Look doesn't support. Fortunately, Apple was smart enough to design Quick Look with an open architecture that allows developers to write their own plugins and support more file formats, which Japanese developer Taiyo used to write two excellent plugins.
The first addresses a serious annoyance with the default Quick Look implementation on folders. If you invoke Quick Look with a folder selected in the Finder you'll get...a picture of the folder icon. Frankly, that's pretty stupid. Taiyo's Folder Quick Look Plugin fixes this by displaying the folder's contents, which is how it should have been done in the first place. Likewise, Taiyo's Zip Quick Look Plugin displays the contents of zip files.
I'm sure we'll be seeing more and more of these expansions of Quick Look in the days ahead, which will make this quintessential Leopard feature that much more useful. Both the Folder Quick Look Plugin and the Zip Quick Look Plugin are free downloads. Place them in your /Library/QuickLook/ or ~/Library/QuickLook/ folders and they should work immediately.