One other reason to add is hardware compatibility issues, specifically hardware that one would expect to work such as keyboards and Wi-Fi.
The Wi-Fi issue can occur either when the computer boots or comes out of sleep, where it shows the Wi-Fi as available but cannot connect. Generally, disabling and re-enabling the adapter gets it connected, until the laptop goes to sleep.
I've seen the keyboard issue on some Acer and Toshiba computers where when the computer is booted using "Fast boot", the keyboard does not respond until it is unplugged and plugged back in. With a laptop, it needs to be rebooted. Disabling "Fast boot" generally gets around this issue with a side effect that Windows 8 must do a full boot when powered on.
Although I haven't seen these issues on fairly new computers apart from the keyboard issue on one Acer PC, I would imagine these issues discourage users of older computers from upgrading to Windows 8.1 when they encounter other people with these problems.
For example, if IT support technicians for a large company are having keyboard and Wi-Fi issues with their personal/home computers after upgrading to Windows 8.1, I can just imagine those technicians daunting about IT headaches of similar issues when discussions arise within the workplace about moving to Windows 8.1...