One of the greatest new features in Leopard is Quicklook.
I had tried a couple "quick view" programs in the past, but they never worked quite the way I wanted them to. They would open at the wrong time, tell me stuff I didn't want to know, slow my computer down...
With Quicklook, all that is in the past. On my Macbook Pro (with 2GB of memory) I was impressed with how well Quicklook performed and used it often with as many plugins as I could find for it.
I was worried, however, that my Powerbook G4 (with 512MB of memory) would lag and run so slowly I would have to reinstall Tiger. I was to pessimistic to believe the rumors that Leopard actually sped up old computers.
I finally, out of necessity, installed Leopard on my Powerbook and held my breath as I pushed the space bar for the first time. It worked, not nearly as quickly as my Macbook, but much faster than I expected.
I now use it everyday, sometimes just to watch its awesomeness in action.