Nobody ever reads those terms and conditions, do they? We just assume everything is kosher when we click on that "I agree" button, because we just want to get on with our day. Of the over one million people who are testing Windows 10 Technical Preview, I imagine a great number of them did exactly that, not realizing that Microsoft is watching your every move with a built-in keylogger.
The thing is that Microsoft was hardly trying to hide this fact from you. They state, plain as day, that the Windows 10 Technical Preview OS would be collecting and tracking information about you during this trial. They'd be looking at "you, your devices, applications and networks, and your use of those devices, applications and networks," presumably in an effort to fish out the bugs and make Windows 10 better. We figured that they'd be looking at your preferences, browsing, search history, device configuration and application usage.
In the newest build of the Technical Preview, Microsoft has now added a very specific line about watching your every keystroke with a keylogger: "[W]e may collect typed characters and use them for purposes such as improving autocomplete and spellcheck features." Why is this a security threat? What's the big deal if Microsoft knows that you type in facebook.com in your browser more often than you're willing to admit? The risk is that the keylogger is effectively also recording your email addresses, usernames, passwords, banking security codes and anything else that you type on that keyboard.
We might be able to assume that Microsoft won't be nefarious with this data, but we can't assume that this data is 100% safe in their hands. All it takes is a clever hacker with motivation to do so, and suddenly the keylogger database is unlocked for their perusal. What if the hacker could tap into the keylogger and see your keystrokes live? That's awfully dangerous, wouldn't you say?
Of course, the Technical Preview was never meant to be utilized on your primary machine as your primary OS anyhow. In this way, it's probably in your best interest to treat any machine running on the Windows 10 Technical Preview as if it were a public terminal in a public space.