I managed to get Leopard Preview DVD yesterday (don't ask how) and installed it on external firewire disk, used it for 24 hours and here's few things I've observed so far, just don't tell Apple

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1) There are very few application which are updated. Some are even older than current, like BootCamp which is 1.0.2, and since it is DP, all iLife apps are same versions except DVD Player which is 3.0-nothing different except UI. QuickTime is 7.2.
2) Finder is exactly the same but little faster (no tabbed windows). Spotlight search window has toolbar similar to iTunes, and searching is little faster.
3) Spaces is a handy tool but a little confusing at first. There is also an option to collect windows from all spaces into one.
4) I haven't tried Time Machine yet, but will soon post about it.
5) Mail has more accurate Junk Filter now and all other features previewed at WWDC.
6) Safari 3.0 loads pages much faster and has an extra button in toolbar colored dark brown which is actually a shortcut to Webclip widget shown at WWDC.
7) Leopard now has a separate app for transferring files and settings from another Mac similar to one you see during installation.
8) Boot time is faster than Tiger. The Mac OSX Startup screen does not appear at all. Just the gray Apple logo and straight to Login screen.
9) Right-Click by tapping two fingers on trackpad does not work after startup, but after unchecking-checking that option in Sys-Pref brings it back. Similarly, Parallels needs reinstall, and PS needs re-activation. Otherwise, it is pretty solid.
I Haven't found any "Top Secret" features so far. I think Apple is keeping them for themselves and developing in-house. I'll keep you guys updated, provided Apple does not come knocking on my door. I could not post screenshots since they are already taken down by Apple from other sites. So just keep reading.