If you're like me and don't have a Leopard Preview, and if you haven't seen the Leopard Developer Application Technologies Overview, you should take a look. There's some pretty interesting stuff coming down the pipe. I like the Calendar store, which lets any application work with the iCal calendar info in much the same way as AddressBook.framework opens up the Address Book. Maybe we'll see some enterprising developer add support for travel time in calendar display?
I also like the Applescript Bridge - an idea whose time came YEARS ago. It's buried under the section titled "Picking Up the Pace of Cocoa Application Development". What a major simplification!
I like that a lot of the new advances in Leopard seem to involve making it easier for apps to work together and share data. My data should belong to me, not to the application I first entered it in. This is as much a usability issue as it is a data safety, vendor lock-in and openness issue.