I've been using leopard since last summer (ADC Select Developer) on my firewire hard drive. It has grown in leaps and bounds since then. I think the biggest thing that would be classified as TOP SECRET (but come on, this is blatantly obvious) is the new Finder and UI. Microsoft trying to copy stuff from Leopard to Vista? Thats impossible due to Vista's release schedule. There's a lot of stuff under the hood, but I cant reveal any of it due to the NDA i signed. Youll see it on the developer pages when leopard finally goes live.
I'm not going to upgrade until MacPorts is compatible with Leopard. I'm a web developer and a lot of under the hood stuff I need just wont work with MacPorts at the moment.
Still, the finder is a LOT faster! Youll like it a lot. Quicklook rocks too. I also like the mandatory unified interface.
Sorry, Children, Leopard's Top Secret Features Aren't Real