The first change is the smoothness. Apple built in a technique called anti-aliasing, which smooths everything on screen. Fonts are easier to read, icons are smooth and as large as you choose them to be. Another key feature is the Dock. A handy feature built into the Dock is that anything you minimize stays there, previewed so that you could continue to watch a movie you minimize in QuickTime, Apple's flagship video technology, or anything else, like a Web page in Internet Explorer.
Files are also easier to use as well.
Instead of navigating through up and down and in circles sometimes, Apple developed the 'column view', which horizontally presents the path, opening each folder in sequence so you can see where you came from and where you're going. It may take a little getting used to, but there is no denying that it is a much faster way to find just about anything. Customization is also a big theme. You can easily drag buttons around to new positions, re-size windows smoothly, and configure every setting under the sun to make it your own. Lest ye of Apple stock shy away, many of the familiar favorites are still there.