If you try to use the new Cortana search app on Windows 10 you must first log into your Microsoft account. And that is not all. After you log in, you must first convert your computer's Windows account from a Local account to a Microsoft account.
The enormous difference between a Local account and a Microsoft account, of course, is that with the latter you are permanently signed into Microsoft's server which can remotely track all your actions.
This is a huge invasion of privacy, especially for people like us who give Microsoft free beta-testing services.
Do not convert your Local account. Instead, disable Cortana (see http://www.windowscentral.com/how-disable-cortana-task-bar-windows-10-preview).
Microsoft should remove this requirement forthwith and let us loyal Windows Insiders use Cortana without converting our account, just as we do the Windows Feedback app. Otherwise, it will simply lose its loyal customers.