I have major concerns with Windows 10.

Firstly, its updates. Some updates are mandatory, including device drivers. In the pre live version, there was a way to turn this option off for at least some things, its being reported that the live version does not have this ability. So your drivers will auto update based on what MS think they should be!

Some security patches are also mandatory, like the "security patch" for windows 7 that included a little feature constantly telling you to upgrade to Windows10.

Privacy. Try reading the MS terms of agreement, they are 45 A4 pages long if you print them out.
Included in here is basically the right to gather pretty much any and all data from your computer, and send it to them, including passwords and encryption keys.

FREE? Forbes magazine reports that Windows 10 will be the last "version" of windows. After this, it will becomes just "Windows" and will turn to a subscription based operating system by 2017. You wont be paying for patches and fixes to existing features and functions, but if you want the new stuff they will be rolling out, then you have to start paying annual fees, and you can bet that some of that new stuff will be mandatory for certain other popular applications and games.

NOTE: Windows 7 support continues until 2020.

I will be sticking with Windows 7 for at least another 364 days.