I can only think of things like the age of your Mac, how much RAM ones has, third party apps conflicting, or third party hacks, but why do people have different issues when they upgrade to the newest OS X. For instance, I don't have issues with WiFi, yet so many others do. With my MBP non-retina, the font rendering if off majorly. My AirDrop is fickle in that I can send from my iPad or iPhone to my MBP, but sometimes not the other way. It works and then it doesn't. I think a lot of casual users probably don't even realize that their update to Yosemite made their Macs buggy and may even go to a tech store to get their Macs fixed. Apple doesn't usually admit to bugs openly so I think most people don't have a clue unless frequently sites like MacRumors. Any thoughts on why we have different performance issues when upgrading to the next OS X?