-- First, delete ~/Library/Java/Caches/deployment.properties
-- Move aside your Java 1.6 directory. The 1.6 preview on Tiger does not work on Leopard.
% cd /System/Library/Frameworks/JavaVM.framework/Versions
% sudo mv 1.6.0 Tiger_1.6
% sudo rm 1.6
At least it comes with better Ruby support and a RubyCocoa bridge.
Update: I don't think I like Leopard. I don't like: the 3D or the 2D look of the dock, the semi-transparent menu bar, Spaces behaviour - you can't have multiple windows spanning desktops from the same application (like two browser windows in separate desktops), the removal of text lists (why not have Fan, Grid and List?), in Quick Look you can go to a page in a PDF or Word file but when you click on it it goes to the first page, and Java support (IntelliJ and others seems to have weird refresh issues and you can't seem to allocate it to a virtual desktop).
There's of course a lot to like though too (tab terminals, RSS reader, better Spotlight, Cover flow, Safari, etc).
Update 2: So the Spaces thing. If you minimise a window, it goes to the dock, change to another virtual desktop, then expand, it goes to the desktop that the window was originally in. As noted in the comments though (and it is in the guided tour), if you activate Spaces (the default is F8), then move the window to a new virtual desktop then it is tied to it. There are two other ways: click and hold on the window and switch to another desktop or click and hold on the window, move it to the edge of the screen, wait, and it will go to the next desktop.
To me, some of the behavior breaks the illusion/metaphor of the virtual desktop and seems unnecessarily difficult. I'd prefer non-click and hold options supported as well (pretty much like other features like copying files, etc).
The networking improvements (non-blocking, built in VNC) overcomes what was probably one of the worst things about OS X.
The IntelliJ issue that I mentioned is logged as issue 16084.