* They have some kind of child monitoring thingy which is on by default. A friend of mine got very surprised when he had installed Windows 10 on his kids computer and a few days later got a report in the mail, detailing such things as when the kid had used the computer, which programs had been used, which sites had been visited and so on. This wasn't something he had activiated, somehow MS had worked out that he was the parent from the log in which is connected to some social platform. So very much not OK.
* The use some kind of swarming protocol, kind of like bittorrent, to distribute updates. This means that they use your bandwidth to deliver their updates to other users. What makes it worse, the people who cares the most about updates are organizations, and they will block that function, so they will not take their part of the load.
* If you run an earlier Windows, it downloads the Windows 10 update to your hard disk, just in case you'd want to install it. This uses a lot of disk space and bandwidth. I suspect the real reason is to provide more sources for the swarming protocol, as a described above.
* Among the leaks released by Wikileaks was that all the major operating system vendors has been approached by the NSA and more or less been ordered to install a back door. This is also confirmed by Linux, who was also approached and went public instead (and telling them that not only is it impossible to hide something in open source, they still wouldn't do it anyway). Neither MS nor Apple has commented on it, which makes it likely that they caved in to the pressure.
* It has a feature (which is on by default) which shares your wifi password with everybody who is your friend on social media. This is supposed to make it easier for your friends to connect to your wifi when they visit. However, it also means that your wifi password is stored somewhere else, outside your control. It also means that a lot of people who you may have as Facebook friends but not really want on your network have it. Your ex? Your neighbor? Some friend of a friend who plays Farmville?
It also messes up any organized strategy for safe friend access. I have a private network with a password only known to me, which goes to my internal network and servers. Guests connect to another network, which only connects to the internet. Which password will MS share?
We also have the entire issue with BOYD. Will they share your work wifi password as well?
* Unique ad ID. You get a unique ID on installation which is then shown to web sites, allowing them to use personalized ads. However, it also allows you to be tracked across sites, so it is possible to see that it's the same person on Facebook or GMail as on f*cked.raw.in.the.pig.sty.com. This is a huge integrity problem.
* They have a feature which allows them to stop certain programs from running if they don't like them, and even remove them. Cloaked under the disguise as anti-piracy, it is bound to harm legitimate use as well (such as using a cracked version of a game to make it work because they've effed up so the originals no longer work). Also, it would be trivial for them to say "We don't like LibreOffice and OpenOffice, they compete with MS Office. Let's block them.".
* They seem to have removed the option of "Don't install updates", only providing "Download and install" and "Download and install later". This is a huge risk, as it allows them to, at will, replace just about Everything in the operating system or on your disk.
* It has tools to redirect "unsuitable" internet sites to other sites. It would, for example, be perfectly possible to use this to block, say, Wikileaks, and redirect to, say, NSA.
Now, all these integrity issues has slowed down adaption of Windows 10, so in a recent press release, MS said that they are going to put these "features" in Windows 7 and 8 as well. Let that sink in. Basically, they say "OK, you won't get Windows 10, because you don't like what we do with it. Well, ha ha, we are going to do the Windows you run as well, so it will be as bad as Windows 10, so you won't have any reason to update!". That stunt is insulting enough to make sure I never update (and leave automatic updates in Windows 7 off...).
Then, of course, there are several other problems, especially for gamers:
* They have removed support for some copy protection mechanism, which means that hundreds of old games no longer works. I'm not talking archeologically old, just a few years.
* Performance. While Windows 10 on an average gives about the same frame rate as Windows 7, it's not smooth. It has a slightly higher framerate, but then it stutters and freezes for a short moment, making it next to unusable in gaming (especially competitive gaming).