1. Windows 8.1 was designed solely for touch input/tablets. Windows 10 is more of a desktop OS. Many tablet navigation features found in Windows 8.1 were removed in Windows 10.
2. Forced updates. What if I want to use my tablet NOW? Oh no, Windows is updating... Please wait a while before you can actually use your device.
3. Forced updates. I don't want to install any updates that will break the system, which Windows 10 updates have been known to do.
4. Wi-Fi Sense. Sorry, I don't want to let the world know my router's password. Windows 8.1 doesn't have this feature. I feel safe knowing my Wi-Fi password will never be shared in Win 8.1.
5. The point of Windows 10 is to collect user data. Everything you type gets analyzed by Microsoft or the OS. They even track the way you type (each person types differently like each person has unique fingerprint) to ID you. Windows 8.1 never does this (if you don't install the updates with telemetry that is...)
So as you can see, there are too many bad things about Windows 10 that I will never use a device with Windows 10 on it... Windows 8.1 for me!