I hate Windows 10 due to the power it takes away from power users. I don't need my computer overwriting my drivers with different versions without my consent, nor do I need Cortana always running when I don't use her. I appreciate being in control of what my computer does, and don't like it restarting whenever it damn well pleases without my consent. Additionally, I like having the ability to pick-and-choose updates.
Do note that my opinions were formed the first month after it came out. I don't know if these issues were worked around in later updates, as I reinstalled Windows 7 when I couldn't take it anymore. The drivers thing was actually the last straw for me. The official Asus trackpad drivers for my laptop absolutely suck, so I kept using official Elantech drivers. Windows 10 didn't like that, and kept overwriting them on a daily basis.
(Also, no operating system should come with adverts enabled by default.)