Windows 10. When your OS no longer respects your choices and you have to fight it every minute, there is something wrong. The creeping invasions on privacy have only cemented my use of Linux
Truthfully, I'm not sure if I would have ever switched over if Microsoft kept the Windows 7 paradigm. But I started my search for alternatives when Windows 8 - already too adventurous for me - came with the computer I bought.
Towards the end of my time using Windows 10 as my primary OS, the realization that the UI is not an inherent component of the OS sealed the deal. As a Windows 2000 fan, I fell in love with the way Chicago95 Debian replicated the look and stability that I had sorely missed.