Honestly, Leopard looks rather disappointing. I mean, it'll be a nice operating system and all, but there weren't really any "Vista killers" that the community had been hoping for. Most of the features are already in Vista, albeit not as polished as Apple's solution (Time Machine being the obvious example). And more of the features that aren't in Vista aren't particularly compelling. Stacks could be nice, but they've been sitting on that for years. I remember reading about them in Bruce Tognazzini's Tog on Software Design (1995)--I guess they still beat Cairo though...
Is there any summary (e.g., bullet points) on the developer features? I'm aware of CoreAnimation and ZFS, but haven't heard much else. It almost seems silly that the big news is yet another way to render HTML on Windows.