i am pretty sure that users will notice a difference. there are actually a few visual differences, but the 're-write' of the OS will make things not only run a lot faster, but take full advantage of all not only all CPU's in the system, but GPU when developers release their updates for current software
This.
My belief is that 10.6 will be become the baseline for the next generation of Mac softwares.
Imagine Safari 5 (if 4 doesn't include it) taking advantage of the tab per process by using Grand Central and OpenCL to power some of the HTML5/CSS animations. All Apple's pro software taking advantage of the GPU, Final Cut/iMovie using the GPU/CPU to encode faster or provide real time special effects, transition effects, so on without any hit on performance.
Hell, I am hoping Adobe would use OpenCL to power their next flash player so that it'll stop sucking so bad on macs.