Well, it's been a bit over ten days since Leopard was released, and almost as long since I started using it. So far, most of my attention has gone to the new Spaces feature, which is a great addition but has a few behavioral quirks in the version included with 10.5.0. They're nothing I can't get used to, given a little time, and I do have Spaces enabled, but I'm hopeful there'll be a few adjustments in how it works in 10.5.1 or 10.5.2.
Others may differ, but I like the new Dock, with its reflections and bright dots to indicate running applications. I've given up on trying to include everything in the Dock that I'd like to have there, and have installed aliases to a handful of apps on one side of the Desktop. I wish that it had occurred to me to do this before, both because it allows me to put more apps where they're easy to get to and because their icons make nice decorations on the Desktop. It also allows me to group them in two dimensions instead of just one, as on the Dock, and I know right where they're going to be if I need one of them. The Dock is still more convenient, but aliases on the Desktop rate a close second.
I also like the newly adjustable grid that controls icon placement on the Desktop and in Windows, the more detailed icons, the unified window appearance, and the inclusion of predefined searches in Finder's sidebar.
As much as any new feature, though, I like the sparkling performance. Tiger was quick, but, with very few exceptions, Leopard is quicker. It also feels quite stable for a point-zero release. Aside from the rules driving the behavior of Spaces, I haven't found any bugs at all so far, and only one feature gone missing, the ability to launch a URL from Terminal using Command-Doubleclick.
My main reason for wanting Leopard was the new version of Xcode, with support for refactoring, and of Objective-C 2.0, with declared properties, iteration over collections, and garbage collection, and that's where my attention is now turning. The rest I'll get around to sooner or later.