Snow Leopard has few evident changes but many subtle ones. It's a trimmed, optimized version of v10.5 (Leopard) which came before it-a full 7GB smaller after install! Some minor features are indeed new, but the majority of the changes are under the hood. To begin with, installation is drama¬free as expected. Upgrades take about an hour, depending on how much stuff you already have installed. Backups are recommended prior to the upgrade, but we experienced no loss of any kind. The rumors of being able to upgrade from OS X v10A are true, but Apple only officially supports upgrades from Leopard.
Snow Leopard doesn't have any of the usual hype around new features, because there are barely any that the user can see. All improvements are backstage, designed to make OS X faster, stronger, and less prone to bugs. That also explains its relatively low cost. Snow Leopard is now fully 54-bit. All key components of the OS, including the new QuickTime X, are 54-bit. While this doesn't impact the end user that much, it is a statement of Apple's foresight. Grand Central Dispatch is another feature that you won't see but that will make applications work better with multi-core processors. The same goes for OpenCL, which allows apps to take advantage of a graphics processor to speed up tasks.
On to what you can actually see: Expose is subtly different with a more even distribution of windows when activated. A nice touch is how it can now just show a single app's windows with a click-and-hold of its dock icon. The dock and stacks have some new functionality; use them a while and you'll notice they play really well with multiple windows open. All in all, this is an upgrade that isn't as significant in terms of features as Leopard, but contains many potentially important advances for the future. Sadly, all this is Intel-only; there's no support for older PowerPC-based Apple hardware.