For me, it’s the multitude of spying vectors that they keep reintroducing, whether those are well-intentioned or not. I also dislike the forced updates unrelated to security, like drivers. I upgraded one ancient laptop to test, but I will probably not upgrade my other three PC’s.
10 is fast, and has a few nice improvements, but not worth the hassle. I’m fine with 7, and even 8.1 with Classic Shell. Unfortunately it appears that MS is sending many of those days collection updates downstream to the older OS versions as well. I have started researching the Linux world again. It’s been five years since I’ve last explored the wild west of linux. :)
#5 My boot time on my laptop that has Windows 10 on it is about 15 seconds to the login screen. I have a 7200rpm hard drive in it. I am not putting Windows 10 on my desktop pc as I hate the Sword of Damocles of Privacy hanging overhead.