Feature: Cover Flow in the Finder.
How it works: Just like album Cover Flow in iTunes, Leopard brings the side-scrolling view to the Finder, allowing your "flip" through your files and see live previews (including paging through mutli-page documents and playing movies).
Who will use it: Everyone at one time or another. When I first heard of Cover Flow in the Finder, like Matt Neuburg, it seemed like pointless eye candy. But like him I'm beginning to think otherwise. Cover Flow makes quickly flipping through a bunch of files to look for something much easier. This becomes particularly important when you're looking through folders you're not that familiar with. So even if you're more inclined to keep the Finder in a conventional view, Cover Flow will still probably come in handy on occasion.