Today I noticed that you can review documents before you choose them in the Open dialog, if only at thumbnail size. After selecting File » Open, select column view mode then select the document you wish to review (single click). You can then see the thumbnail preview of the first page of your document. This isn't news.

However, now click More Info under that icon and the Finder's Get Info box appears. If the chosen file is a QuickTime-viewable clip, you can preview the file. If it's a document, hover the mouse over the document icon in the Get Info window, and click the arrow key buttons to go back and forth through the document's pages.

It's a simple way to quickly check the contents of a document before you open it.

[robg adds: You can do most of this in 10.4, too, but there's no ability to really preview the document.]