I posted quite a bit in an old thread on the topic:
http://macosx.com/forums/bobs-place/229437-what-features-would-you-like-see-leopard.html
In brief:
More multithreading in almost all apps, especially the Finder and Safari.
Major bug fixes in the Finder and Spotlight.
Better Smart Folders that are fast and don't randomly refresh when you're trying to use them.
Dynamic folder icons and better dynamic file icons. <- This one IS in Leopard, at least regarding files. I'm not sure about folders.
User-definable file tags, sort of like in Gmail.
List View columns for any and all Spotlight tags, so I can, say, sort images by width in the Finder.
Personally I'm a little wary of the ZFS hype. I'm not sure it presents any real advantages for a desktop OS. Seems to me like it's geared more for servers, and is
a bit over-zealous. It sounds awesome, don't get me wrong, but it seems like the real-world benefits are mostly theoretical.