(Which your PC may or may not be installing right now.)
Who is affected
If you primarily used the "normal" Windows desktop, in Windows 7 or 8.1, you will not be affected by this. Microsoft changed up some things for you, but not a whole lot, and if you didn't like the Start screen in Windows 8.1 the Start menu now works more like you're probably used to.
If you mostly used modern fullscreen apps in Windows 8.1, and stayed in its tablet interface on a laptop or desktop PC, you are affected.
What it is doing
Windows 10 forces all non-tablet users into the Windows desktop. There is a way to turn on "Tablet mode," but it will not work if you have an external display connected.
Windows 10 also lacks many of the features associated with 8.1's Modern interface, such as the Charms bar, the task switcher list you could get to by mousing into the upper-left corner, the Immersive Browser of IE 11, and the horizontally-scrolling Start menu. If your app icons were carefully laid out and grouped in Windows 8.1, they will be gone in Windows 10, and replaced with apps that you didn't choose to install but came with the upgrade. Also, on the desktop focus doesn't follow the mouse, so you can't (for instance) scroll in a browser window while chatting in Skype.
Microsoft recently forced everyone who uses Skype on Windows to download its Desktop version, even though the Modern app was working fine, and claimed through error messages and install dialogues that your Skype experience was being improved. Like with banner ads all over the place.
(This link explains how to disable the ads in Skype.)
This is basically what Microsoft is doing to everyone who grew to love and rely on the fullscreen, Modern interface. You can't get it to work on certain systems, it doesn't work nearly the same, and in general everything is more cluttered and intrusive and there are more ads. If this is not to your liking, you may want to consider not upgrading.
Apparently the people who want everyone's computers to work like they did in 1995 won, and we can't have nice things anymore.
We are downloading Linux.
We have wasted the last three years, and all the time we spent learning in Visual Studio.