I am a Mac user, the only true technical reason to upgrade the Mac OS since 2000 or so was when they switched to the x86 platform. They incentivize upgrades more by by outright dropping support for old hardware than the relatively minor features they add in what basically amounts to a yearly service pack. I just upgraded to Mountain Lion for the hell of it and I wouldn't know I had upgraded if I didn't know what to look for.
The majority of what you listed in OS X has been implemented in some fashion in Windows (task bar upgrades, task switcher upgrades, voice dictation), maybe not as an OS release but as a free download (e.g. Skydrive / live as opposed to iCloud).
Also, as far as I can tell, there was not much to "fix" in Vista. Most of Vista's problems were due to terrible drivers which improved over time. Outside of that, the only other real issue I remember was UAC so I doubt much time was spent on that.
As for Windows 98 vs XP, it's like I said. Windows XP was 100x more stable than Windows 98 because it ran on the NT kernel and businesses knew there were huge productivity incentives to upgrade - similar to the OS9 vs OS X update. Neither OS has seen an update that had near the quality impact since.
Finally, I'm on Windows 8 now and I have to disagree about Metro. I don't think it's anything revolutionary but it adds much more value than it takes away. Exponentially more useful than half-hearted stuff like Launchpad.