The EFF has a great piece on the lack of privacy in W10 and more importantly the loss of any form of control.
They note:
The trouble with Windows 10 doesn’t end with forcing users to
download the operating system. By default, Windows 10 sends an
unprecedented amount of usage data back to Microsoft, and the company
claims most of it is to “personalize” the software by feeding it to the
OS assistant called Cortana.
Here’s a non-exhaustive list of data sent back: location data, text
input, voice input, touch input, webpages you visit, and telemetry data
regarding your general usage of your computer, including which programs
you run and for how long. While we understand that many users find features like Cortana useful, and that such features would be difficult (though not necessarily impossible) to implement in a way that doesn’t send data back to the cloud, the fact remains that many users would much prefer to opt out of these features in exchange for maintaining their privacy.
And while users can opt-out of some of these settings, it is not a guarantee that your computer will stop talking to Microsoft’s servers. A significant issue is the telemetry data the company receives. While Microsoft insists that it aggregates and anonymizes
this data, it hasn’t explained just how it does so. Microsoft also
won’t say how long this data is retained, instead providing only general timeframes. Worse yet, unless you’re an enterprise user, no matter what, you have to share at least some of this telemetry data with Microsoft and there’s no way to opt-out of it.
Thus Microsoft can track your every move. Worse however is that Microsoft single-handedly can block you emails to those who use MS emails such as hotmail. Thus not only do they have potential access to any and all of your emails they also decide who you may communicate with. And you will never know this. Namely if MS decides for reasons only known to them that the sender's email is unacceptable they then block it. The MS email user will never know that it was blocked. The blocked email sender must go through multiple hoops to the extent of threatening litigation to free up the connection.
Overall in my opinion and based upon my experience the world would be a lot better with an alternative, soon!