Most reviews notice that the installation routine has changed. Instead of the installer offering you several different types of upgrades (or even a complete re-install), the installer will automate it all for you; you only need to enter your password, and get going. In fact, you can do this right from Mac OS X itself, you no longer need to reboot off the DVD. The installer will place incompatible applications in a Incompatible Software folder.
The installer itself has become a lot smarter too. Instead of installing all those printer drivers, you can tell the installer to only install drivers for the printers used on your Mac and/or currently on your network. Rosetta is not installed by default, but can be tocked off in case you need it. all language translations are still installed by default, but now only take up 250MB of disk space.
The promised reduction in overall disk footprint is definitely real: users are reporting massive disk space increases, of course depending on how many applications you had installed. This is achieved by, among other things, the removal of PowerPC code from the system.
The big news, which came out today, is that the Snow Leopard upgrade disk will upgrade your copy of Tiger just fine too; Leopard is technically not required, and Apple did not put in any checks either. Of course, this upgrade method is unsupported, but since Erase & Install is still supported, Tiger users can rejoice - they will not be forced into buying the box set. Intentional or not, great move by Apple.
Reviews all note the improved Exposé and Stacks functionality, and apart from the improved installer, those are two of few noticeable graphical changes in the operating system. Exposé is now linked to the dock, so holding down an icon int he dock will Exposé its child windows. Stacks have been made actually useful by being scrollable and drillable.
The transition to full 64bit means that the Finder has finally been rewritten in Cocoa (I will remain recalcitrant by pronouncing it Coco-a), meaning full 64bit goodness. This rewrite makes the Finder ridiculously faster than it was (and let's face it, that was sorely needed). There's a new feature as well: live file previews, so now you don't even need to Quicklook to see a file.
Quicktime X is the newest version of Apple's multimedia layer in Mac OS X, and apart form the new interface for the player (which I need some face time with before I say anything about it), it has been completely rewritten to hook into several core systems of Mac OS X, such as Core Video, Core Animation, and Core Audio.
Technologies like OpenCL and Grand Central dispatch are also major additions to the operating system, but it will take some time before the 3rd party ecosystem will take proper advantage of them.
There are, of course, a number of compatibility issues, but that's usually the case with Mac OS X upgrades. Performance has improved across the board, the reviews say, so it seems like there is very little reason not to shell out the mere 30 USD to upgrade to this latest release.