Here's another feature in Mac OS X Leopard, I discovered it by accident: if you have a file (or folder) highlighted in Finder, hit the space bar, and a huge preview will appear. This is neat because you can navigate a lot of files with the arrow keys and instantly get a full-sized preview of any kind of file, picture, text, Word, PDF, etc. And you don't have to have the app open. (And even if you do, opening a file in an app takes longer.)
This is not the same as coverflow, and may be more useful, because the files are not covering each other.
If you select more than one file and hit the space bar, you will get a slide show which you can leaf through with the arrow keys. I can really use this.
Update: in the interest of balance, I should also report that Leopard is less stable on my Mac Pro than Tiger was. Tiger very rarely crashed, Leopard so far has crashed at least once a week, mostly when I wake up the machine. Poor show.