What are the major differences between Leopard (10.5) and Snow Leopard (10.6)?
I remember watching the keynote when Snow Leopard was introduced: Steve Jobs pointed out how fast Mac OS X was being improved and new features were added. Several major upgrades have been shipped, while Microsoft took years to ship Vista.
From the beginning, Snow Leopard had the focus on improving the overall system performance and optimizing already implemented features. Also, cleanup was required as PowerPC support was dropped.
Because no major end user related features have been added, it's been widely assumed that this is why the price dropped from $129 to $29.
Changes:

support for 64-bit applications (you can also boot the kernel in 64-bit mode if supported)
Grand Central Dispatch helps to get more out of your multi core processor.
OpenCL offers GPU hardware acceleration for any application.
PowerPC support was dropped.
Microsoft Exchange Support for Mail, iCal, Address Book.
QuickTime X (full screen view, http live streaming...)
Safari introduced Top Sites, Cover Flow and sandboxing for plug-ins.
Anti-malware alerts.
Finder rewritten in Cocoa.