I didn't care for the UI at all which is the most common complaint about it. But it was the least offensive complaint to me, the existence of the Microsoft Store was had me looking for the exit. I guess I overestimated Microsoft's competence because the store remains an irritating tumor that hasn't yet metastasized.

Unpopular opinion: Windows 10 is worse. You could still control updates and uninstall or never install the telemetry packs in 8/8.1. And the UI problem could be solved with 3rd party tools. Windows 10 had the spyware baked in and got more annoying over time. But poor adoption of 8 and Microsoft abandoning it meant that in practice driver and software support for 8.1 often disappeared before even Windows 7.