One of the little improvements in Leopard that hasn't got much press but still makes me grin is renaming files with extensions. In Tiger when you hit enter to rename a file in the finder, the entire name was highlighted, including the extension. When renaming you were forced to go through the additional four keystrokes of the period and file extension. (i.e. ".pdf") In Leopard the OS does not highlight the period and file type extension. It just highlights the name. This makes the process faster and prevents the problem that occurred when I would previously make a mistake on the extension and cause all sorts of problems when I went to access the file in the expected application.
This is not a huge feature but it sure is nice.