General reliability. The point of owning a Mac is to not have to reboot it regularly. There have been times in the past between OS X updates where I’ve gone weeks to months without a restart. With Yosemite, I typically have to reboot my laptop at least once a day, and my desktop every few days of use. Rene Ritchie of iMore notes that no two machines he owns have the same issues, even. Apps will spontaneously quit for no reason, sometimes in cascades, requiring a restart.
My MacBook Air is far too frail to compensate for instability issues arising from Yosemite, so I continue using Mavericks.