In particular Toyota car ads in the music app.
I did like the UI improvements, the notifications system etc, that switching between new/old apps didn't feel jarring anymore.
I think the kicker was the github project calle dunfuck windows 10 or whatever it was, where you ran a bunch of batch scripts to remove/kill off the monitoring services, and I just didn't want to have to worry about that.
Also the fact that on my other machines, the nagging to upgrade to windows 10 is constant, I had to go find/remove some random windows update patch from June/July to get rid of gwx.exe.
edit: lastly, the licensing terms of the operating system itself, IANAL, and I couldn't tell what rights I had to the o/s in the future.