I've now had some more time to play with Leopard, and I've found a few features that aren't immediately obvious, at least they weren't to me. So, in no particular order:
The slideshow feature in Finder isn't gone. It looks like it has been replaced by Quick Look but holding down the Option (Alt) key when opening the context menu brings back slideshow. I like this much better for viewing a set of pictures or browsing through some movies.
Some Apple applications have special support for Time Machine. This must be described somewhere prominent but I missed it for a good while. Just click on the Time Machine icon in the dock while Mail or Address Book are in the foreground... I like the feature but I don't like the way it is invoked. So far, clicking on an icon in the dock made the application active. Nothing else. Now, however, the result of clicking on an icon in the dock becomes contextual, depending on which other application is active.
Preview now has support for annotations and other markup. Just use the customize toolbar function and have a look around.
There are a few new visualisers in iTunes. And they look good!
Last but now least, change the timezone in one of the dashboard world clock widgets and observe how the hands are animated. I guess, we'll see a lot more gratuitous animations now that core animation has been released.