I'm not trying to bag on anyone here. But all the complaining about Yosemite makes me wonder if it's just the users, not the OS.
I have several Macs running Yosemite. No Finder slowness, no Mail issues, no Safari crashing, no WiFi disconnects, no Preview slow-downs with PDFs (though I generally use Acrobat), no Bluetooth disconnect issues.
The only issue I have with Yosemite is the Notification icon disappearing from the menubarand I KNOW that it's due to a conflict with Bartender app.
Most all of the Macs I own are maxed out on RAM and have SSD drives, so I suppose that could be a reason. I also don't run any sort of disk utilities or 'cleaning' apps. In general, I don't run a lot of background apps either.
The point is, why do so many not have any issues, and so few do?