I will preface this by saying that I was a Mac user and developer for over a decade until very recently...
To your Classical MacOS I reply Windows Me.
Sorry, will have to call this one out. At the time that classical MacOS was around, most users were either running Windows 98 or Windows 2000. Few people ran ME... I worked in the industry and I think at the time for every 10 Win2K boxes I saw, I saw maybe one ME and 15 Windows 98. Noone saw a compelling need for ME so they didn't upgrade. And early versions of OSX up to Jaguar (10.2) were horrendous messes under the hood. The interface was gorgeous but some of the APIs were kludged together wrecks.
Now, I'm not saying that Windows was much better, at least on the 98/ME track... but the Windows 2000 API cleanup actually produced some really nice results. This was at the same time (roughly) as Mac OS9 which was one of the most awful kludges of an OS I think I've ever had the displeasure to work on. It was obvious that OS9 was created in a place where OSX was getting all the attention, but even OSX wasn't really properly clean until 10.2... and even then it was slow. Have you ever used 10.0 or 10.1 on hardware of the time? It was horrifyingly slow to do anything and the only thing it really had going for it was the interface.
Speaking of the interface... seriously... Finder is significantly worse than Explorer in terms of threading, resource utilization and stability. You want to see a kludged mess, check out Finder circa... well... any version of OSX actually. Of course, I've recently abandoned the Mac platform for Windows 8 for various reasons so I can't speak to Yosemite... but every time I work on a machine running Yosemite I just feel like the entire OS is going in a direction I don't appreciate.
Need I compare OS X & Vista? Windows 8?
Vista and Windows 8 both had huge improvements under the hood. Windows 8 in particular has gotten a bad rep simply because it has a UI that people find really polarizing... but it's seriously a fast and efficient OS that really takes advantage of the underlying hardware. It actually is a better operating system than the much-vaunted Windows 7 (which was itself an improvement over Vista) but most people never get to see it because they get hung up on the UI.
I have played with Windows 10, and I like it. I run 8.1 on my computers today but will switch to 10 when it comes out. That's not to say it's a fundamentally "better" operating system than OSX... but for my needs the priorities are all screwed up in OSX. They're both modern, stable and secure operating systems... and if that's all you need then great. However, running the same applications on both platforms does show the weaknesses of OSX; memory management is of questionable value in OSX and the storage management is kludgy at best.
Windows has been faster than OSX on the same hardware since Windows 7. I know; I've always had a Windows Boot Camp installation on my Macs. My last Mac is still a good one (2012 15" MBP) but has now been surpassed significantly. It used to be that Macs had a 5 year lifespan whereas Windows had a 3... that's one reason I liked Macs. Nowadays, not so much... and it's not changing workloads that are killing it but rather the overly heavy APIs and core problems with OSX that just don't scale quite so well... so new versions get heavier and slower on the same hardware.