If you haven't seen this roundup yet, Matt Gemmell's huge list of everything Leopard can do for developers is pretty amazing. It's a bit on the technical side, but worry not-- about 1/4 of the way down the page you'll think that this is a list only for developers, and if you keep scrolling you'll start to see that (somewhere around the "HUD windows" part) Apple has broken open almost all of their interfaces to developers of all makes and models. Everything from menus to date formatting is updated in Leopard's developer applications, and there's lots of "freebies" that will make even smaller programs better-- icons and images, an image editor, and a built-in grammar checker can all be easily implemented in any Leopard apps.
As hot as this operating system is (and yes, despite the problems that folks are having with it), the really good stuff is yet to come. When talented programmers and designers get their hands on these tools, then we'll really see why it's so great to be a Mac user.