This update really took support for the multi-core machines to the next level by improving multi-threading support, and extending a lot of the updates granted to OS level apps in Tiger to the developers for their apps (aka 64-bit memory space.)
There were still a lot of OS sections in Tiger that were big bad single threads, aka Finder. Leopard Finder is multi-threaded now, likely a whole heck of a lot of Leopard is multi-threaded now, not just the choke points.
If it was just a GUI update, a coverflowed Tiger Finder, would likely make you scream.
Haven't played around enough, and I'm used to it, but the GUI lag most Windows users comment on should be reduced.
