The Register points out that by default Windows 10 will upload an encrypted version of your wifi password to Microsoft and anyone in your contacts will be able to use your wifi. Now, that’s a nice feature and lots of people might find it useful — but it should really be opt-in rather than opt-out, even though that would drastically reduce the take-up. Windows Phone has apparently been doing this for some time, but no one has noticed, because no one actually uses Windows Phone.
By default, your Outlook.com and Skype contacts will be able to use your wifi, assuming they’re also using Windows 10. If you opt in, then so will your Facebook friends.
The feature is called Wi-Fi Sense, and it’s relatively hard to opt-out — you have to change the name of your wifi network to have “_optout” on the end. I think I’d rather opt out by not installing Windows 10, and not giving my wifi password to anyone who is running Windows 10.
Even worse, it’s not just your network that’s shared (Windows doesn’t know which network is yours, after all); it’s any password-protected wifi network you connect to. So you’re potentially sharing the passwords of your employer, friends and family, not just your own password.
Edited: It appears that The Register is being needlessly alarmist. It’s hardly the first time, and I should have checked more thoroughly.
Arstechnica has a more sensible version of the story. Wi-fi sense is opt-in in Windows 10, and you have to opt in for each network. However, you should still be careful — for example, Gmail adds everyone you email to your address book, so if you since your Gmail address book with your Outlook.com contacts, then everyone you’ve ever emailed will be able to use any wifi network that you’ve chosen to share. And certainly don’t share your employer’s wifi network.