I think many of you guys need to watch the Mac OS X State of Union video.
Go to
http://developer.apple.com/ and sign-up for for the free ADC Online Membership (If you haven't already.) Then go to
http://developer.apple.com/adconitunes/ and log-in. If everything goes well, iTunes will open-up a secret section of the iTunes store with some videos available. Download the "Session 000 - Mac OS X State of the Union" video.
Many interesting tidbits can be found about Leopard in this video that I haven't seen mentioned anywhere.
There's a nice demo that shows that Core Animation will bring concrete improvement to the UI. It's certainly more than an easy way to access OpenGL. The demo shows how easy you can create a grid of picture elements (for a picture album) that will smoothly rearrange itself according to the sort-order if you remove, add or rename an element for example. Core Animation in this case will take care of automatically animate thumbnails moving around to their new location (reminds me of the Nintendo Wii photo album grid view).
Then, there's a part where they discuss the transition to Resolution Independence. Personally I think many people underestimate what it means.
First it means that Apple has to vectorize every UI elements. They'll have to modernize and reorganize the UI back-end, which despite some visual changes over the years, has been stuck in a very archaic Extras.rsrc resource file, dating back to the ResEdit days. Leopard hopefully will replace the Extras.rsrc file with something more modern and modular. While they are doing this they might as well make some big change in the UI look itself, or at least unify the interface.
Currently, the Extras.rsrc file has to contain 2 sets of bitmaps, one for the Aqua/Blue elements, and the other for the Graphite theme. With vector elements, this wont be needed as it's trivial to re-color vector images. But then why stop at two themes? Why not give the user the ability to re-color the whole interface to suit their mood? Maybe you could optionally tie the color of the interface to outside temperature? Or make a window glow in red when it requires your attention? Tons of new possibilities will be opened by the move to Resolution Independence.
I've seen many comments saying that Apple won't change the UI because we would've known by now because of the time required for developers to make the transition to a new UI.
Here's what Apple says to developers regarding Res-independence.

Be ready by 2008? Hopefully that doesn't mean Leopard will be pushed back to 2008...
I'm not sure what it means though, will we see cosmetic glitches with apps that are not ready by Leopard's launch?
If anyone has a theory about this "Be ready by 2008" statement, I'd like to hear it.
If problems occur because of the transition to RI, Apple will have to show us that it's worth the pain. I expect Apple/Jobs to make a nice demo showing how nice it looks, and how useful it can be.
While many seem to regard RI as simply a way to keep a constant UI size with high-resolution monitors, it will have other uses. Note that the slider in Quartz-Debug not only shows DPIs, but also the magnification factor.
The ability to blow-up the UI elements to a huge size will be pushed as a feature, not as a side-effect of mismatching the DPI of your monitor. Pros will care about setting the right DPI amount, while others, like people with mild to severe vision problems will see that as a God-send. And no the Universal Access Zoom feature doesn't cut it for now, and setting an LCD screen to an intermediate resolution is a clunky solution that result in distorted images.