Jaguar to Panther (to Tiger)
Having started out with Jaguar and just moved to Panther, I see a heck of a lot of improvements across the board: everything from a smoother looking user interface to Expose (the main reason I bought a Mac in the first place), built-in and supported X11 window system (for running my beloved OpenOffice.org), etc.
From my standpoint, having been aware of MacOS X since it first came out but not owning a Mac until this summer, I see the situation as: this was a completely new system for Apple, a huge departure from their "classic" MacOS. It took a couple of releases to iron out not only the bugs but their own concepts of how the OS should look and function. 10.2 (Jaguar) was the first "truly stable" release of what began in 10.0. It had been fixed and tweaked and optimized to make it run well on a wide range of systems. I see Panther (10.3) as the first "major upgrade" to the stable base, filled with lots of usability enhancements, visual improvements (eye candy), and incredible new technologies (sorry but regular old Cmd-Tab task switchers just don't cut it anymore).
Tiger looks to be an improvement but mostly under the hood -- developer technologies like CoreImage and CoreVideo for example, plus general improvements in the bundled programs. But now that I'm up to the Panther level, I have much less anxiety about when Tiger will be released. I'm really quite happy with 10.3.5; it's got everything that I got into the Mac world for.