They are always watching everything you type. They can see what ailments you have if you look the symptoms up on WebMD. They can access private messages that you've typed to a friend or coworker. Yes, even your bank password is sent to the employees at Microsoft. Think about all the governmental organizations, hospitals, private psychologists, and everybody else with non-disclosure agreements in place who will be making the switch to Windows 10.
Microsoft owns Pando Networks, famous for the malware/botnet Pando Media Booster. Windows Update in Windows 10 utilizes P2P by default, most likely the same P2P technology used by Pando Media Booster. Essentially it turns your PC into a zombie host server, wasting your bandwidth serving updates to other computers. This also means Windows Update might be utilized later to spread malware through the P2P system via 0days.
You cannot turn off "Telemetry" unless you own Windows 10 Enterprise Edition. This means your computer will continuously leak unspecified information to Microsoft and there's no way of turning this off.
You cannot turn off "Real-time Protection", aka Windows Defender. Even if you do, it will turn itself on again after a while. This means you cannot prevent your PC from sending random files and information to Microsoft for "analysis".
Windows 10 start menu will have "Content suggestions" which are on by default, which means you'll be seeing ads.
Windows 10 "Wifi Sense" will be begging for your Facebook account details so it can get to know you better. It will also be sharing your Wifi password with all your Facebook/Skype/Outlook friends, their friends, as well as Microsoft.
Why run something proprietary -- something that literally locks you down and spies on you -- just to break it semi-open so you get or get rid of the features you want/don't want?
Why wouldn't you just choose freedom instead of taking the offer of living in a jail?
Escape the botnet. Install Mint. Achieve peace of mind.