* Generate personalized ID for strong and easy person's identification
* Collect keys of your private Wi-Fi networks
* Collect your real name, your nickname, download your contact lists and phone numbers
* Keylog everything you type (despite beliefs of certain individuals, this was not a "feature" exclusive to Technical Preview)
* Index the files on your drives
* Use and manage your Facebook account
* Use everything of above for ADS within the Windows itself (nag screen in the Start menu and commercials in the Windows Store)
* Use everything of above to sell you out to NSA, CIA and FBI
One of the most annoying changes I should also mention are forced Windows and driver updates for non-server/non-enterprise editions. The user tracking is also cannot be turned off in non-enterprise editions.
More "happy" news:
http://imgur.com/gallery/8OJ4t? - Windows 10 is going to invalidate your license if you'll do any changes to the hardware and will force you to buy entirely new license.
http://www.myce.com/news/76883/ - Microsoft is going to apply invasive updates that will render potentially pirated software, games and Chinese hardware clones non-functional.
This is the perfect chance for all of the Open Source community to kick Nadella's company right in the hairy appendages. For ReactOS, is a perfect chance to become the better Windows.
P.S. If you are going to install Windows 10 - think twice. Because, once you'll do that - your PC is no longer yours, by definition.