well in short ( have to run to work)
It is a massive "spy" app. there is a guy, dont have his name right off the top of my head, but he made a small app that you can run that will run off about 27+ win10 built in apps that "watch" and "monitor" you with one click. When you get win 10, or install it, these apps are all turned on by default. There is way much more problems with win10 than that, but its a good IMPORTANT start to let you know why *I* do not use it.
and from the posts here, and many other places on the net, W10 is very problematic with games as well as apps such is BBx (did you notice that without jumping through hoops, you do not have access to your bios anymore???
There is a bonus though, they did bring back DOS LOL.
here is a good read
Another big one for me, lets face it, WIN10 is designed with touch screens in mind - as well as tablets. Thus the metro starting screen and desktop. Everything about it screams touch screen.
I'll keep my win7 - that OS is really rock solid.And it runs very smooth with my gaming win10 has gotten so big, all but the most serious geeks, haven't a clue what's under the hood
Again, just my preference. Currently, with the exception of the big tower I use for gaming, everything is xp pro, x64 and x86. The only reason it is not on the tower is because of driver issues.
that is just a small percentage of the nightmares I experienced with w10.
NOTE TO OTHERS
this post is not meant to deter others from W10. I encourage people to use what they like and what they are comfortable with. These are just my preferences, and views through my personal experiences that allowed me to make an informed decision. Windows systems, like linux distros will always have an eternal debate on which is better. It all really boils down what you are doing, then find a system that matches.