One main problem I’ve seen is compatibility with hardware just a few years old, especially Wi-Fi problems on laptops (example). Do a Google search on “Windows 8.1 Wi-Fi” and there are pages of results with problems, including ‘How to fix’ articles that rarely work.
For example, a work colleague upgraded from XP to Windows 8 just after its launch to avoid the April '14 deadline. Ever since the Windows 8.1 update, the laptop will not connect to Wi-Fi until the adapter is manually disabled and re-enabled in the Network Connections list. Using the laptop switch or ‘Airport mode’ to turn it off/on does not work and this issue returns each time the laptop goes to sleep, hibernate, reboots, etc. The adapter is discontinued by Intel and they (nor Toshiba) provide any driver for it, i.e. the driver page says to use Windows update which just says you have the most recent version installed.
I also had the same Wi-Fi issue with my laptop and a desktop PC after upgrading to Windows 8.1, but at least was able to get updated Wi-Fi drivers to fix the issue on both.
As Wi-Fi is pretty much essential on a laptop, it wouldn’t surprise me that many are put off upgrading to Windows 8 when they hear from others with Wi-Fi issues or other hardware compatibility problems.