After trying every OS X from Snow Leopard to yosemite, yosemite was by FAR the worst. Unlike many people I always do clean installs of every upgrade of OS X. Many people have had great issues such as annoying lags because they upgraded without doing a clean install. I did that once and immediately saw the difference, so I never did it again. That being said, yes there are exceptions when even a clean install might cause problems but that may be a problem within the downloaded installer itself, better download it again and try that one, or it might be the computer in question even.
My problem with the flat, lifeless, weirdly transparent yosemite (other than the design) is the degradation of the easy usability of the OS. In every other OS X, when I enlarge a Finder window, it STAYS enlarged every time I open a new Finder window, even after rebooting, it remembers. But not in yosemite, no sir. I have to enlarge the damn thing every single time I open a goddamn Finder window, and I've downloaded several installers of yosemite and installed them on several Macs, so the bug is not related to a specific installer, and it's STILL NOT FIXED IN 10.10.4. Another degradation in usability in my opinion is the fact that I have to tap on option in order to maximize a window. I never liked using OS X in fullscreen mode, and I could never stand having to wait for the window itself to "transition" into fullscreen. Why do I need to do an extra step just to maximize a window that keeps showing up as small as 600x600 pixels?? Those are HUGE usability flaws that tend to become very annoying very fast. They just make the experience worse. An ugly thing about yosemite is its "choppy" way of filling a copy bar or any other bar instead of the fluid way that was present in OS X. Since when is a choppy way of doing things more enjoyable than fluidity? I like how people claim that Mavericks felt "chunky". I have had almost 0 problems with Mavericks ever since its initial release. Way less issues than I had in Mountain Lion that's for sure. And yes I believe Mavericks comes right after Snow Leopard in terms of stability. As I said before, make sure you do a clean install and you'll see what I'm talking about. In Mountain Lion I had all sorts of small, annoying bugs, just like in yosemite. In mountain lion I kept having tons of problems with the mouse and trackpad and it just didn't feel as stable as Lion did (yes that's right, I used Lion for well over a year and it was just great. Only when I saw how stable Mavericks was did I dare upgrade (after the fiasco with mountain lion)). Mountain Lion is fine now, but not as good as Mavericks, and Snow Leopard's just too old and outdated, so I stick with Mavericks. I can't really remember all the yosemite bugs because I've switched back to Mavericks since before 10.10.2 was released and never looked back but I remember that I had connectivity issues with my Magic Mouse. I was beginning to hate OS X (something I never thought I would ever say), then I switched back to Mavericks, I immediately remembered why I loved OS X in the first place and really enjoyed using it again. All these new and almost useless features are making the OS less and less stable, and with apple releasing a new OS ever single year it's just getting worse and worse. el capitan might seem fine now but who knows how it will actually turn out in the future.
When microsoft released windows vista, in my opinion the most beautiful OS one could think of as far as design goes, Apple DID NOT COPY THEM! They released Snow Leopard and my god what an OS that was! It worked unbelievably good that people still swear by it to this day, even though (let's be honest) it looked 10 years older than vista as far as design goes, but who gave a crap? NO ONE! BECAUSE IT WORKED! Now, microsoft have taken another drastic change in their design, but this time, when the OS actually looks disgustingly bad, sir tim cook and his little monkey jon ivy decided it was a damn good idea to copy one of the ugliest and most unreliable products in the world, even better, they mixed elements from windows vista/7 such as transparency with the flatness of windows 8, making one of the ugliest and most confusing operating systems ever created when it comes to design. Now I admit I am ranting, but I waited a very, very long time to get my mac and then apple started destroying everything that was good and lovable about it, without giving you the option of using the version you like best. Don't tell me "apple's not forcing you to upgrade" because they actually are. I need a new mac at the moment and it's gonna come loaded with yosemite most likely without the ability to downgrade even on machines that originally shipped with Mavericks or even Mountain Lion. Plus you can use Mavericks for so long, there is a day when the applications you use will demand that you upgrade. In other words, Apple KNOWS you're gonna upgrade eventually. I use professional applications such as Pro Tools, Sibelius, Premiere Pro and Adobe Acrobat, and I will say using them on Mavericks has been a pleasure compared to using them on yosemite. And I'm talking from the perspective of someone who has used both Mavericks and yosemite on 3 different Macs.