QuickLook for instant-previewing files with the spacebar is my favorite feature of OS X 10.5 hands down. Encouraging news, then, is this Apple patent dug up by Apple Insider which could add QuickLook to the cursor, enabling system-wide quick peaks or contextual choices for everything in the OS. The patent sounds like it's basically bringing Aperture's monocle view for viewing full-res areas of photos instantly (which is also amazingly useful) to the OS as a whole—hovering over an icon could spring a bubble with four app choices to open that file, or provide other info on what you're looking at.
Apple also seems like they're trying to patent those horrific Snap previews you find on fine blogs everywhere—that blow out a preview version of the webpage being linked to when you hover. That one we can do without. [Apple Insider]