Snow Leopard has major new features that are more significant than anything in Leopard.. But it's under the bonnet stuff...
But also Grand Central, feature to make it easier for developers to use multiple cores.
And OpenCL, to let you use a graphics card to do general computing tasks. Use it's computational abilities for more than graphics, but video encoding, imaging editing, you name it. Because GPU's are designed to handle things in parallel, for some tasks it's dramatically faster than a CPU. This is a big feature.
Both these require developers to support them though.
I don't know about anyone else, but I'm a lot more excited about OS X 10.6 than the new iPhone. The 3G iPhone is out in a few days, and I'm not super excited, but I'd love to see more of 10.6. I was pretty bummed at the little attention it got at WWDC. OS X getting smaller, applications shrinking, ZFS, better use of 64-bit processes. Am I the only one?