So, like I mentioned before, I played with a beta of Leopard. First off, it's obvious why they didn't release it yet, it's definitely not quite ready for prime time. In fact, I have serious doubts that it will be ready to go in October, I wouldn't be surprised at all to see it not come out until Macworld Expo 2008 in January.
Visuals
The new 3D dock is neat, but it's a novelty. I don't like the way running apps are noted. Now you get the black triangle, in Leopard, you get a shiny reflection. What would be cool is if apps that aren't running were placed towards the back in the dock, and apps that are running are in the foreground.

The transparent menu has to go. Period.

The extra shadowing added to whatever is in the foreground does help you figure out what window is currently active, but they need to come up with a less distracting redraw when you're switching from window to window.

The Unified interface looks ok. I've been using Uno for quite a while now to remedy Apple's interface schizo interface design of the last few years.

Spotlight
Finally usable. Wicked fast. Type in a word and bam, it populates with results from all over your system, including the definition of the word you type.

Other Stuff
The Help menu now searches for items in the menu and when you are looking for one, say the "Bounce Back to Sender" option in mail, you click on help and a spotlight like search bar drops down type in bounce and it quickly populates, hover over the results and it opens the menu in question and draws an animated arrow to point it out. Pretty neat.

Spaces is pretty neat.. works as advertised.

QuickLook will be really nice once plugins for all the major filetypes are done. Really sweet.

Coverflow view in Finder. Has potential. Will take time to see just how useful this is.