Account security is always important, but that's especially true when you're talking about security cameras. These devices offer a window into your home, and you don't want anyone else peering through them. Camera hacks resulting from lax password security often make the news, and Google is looking to avoid that with a new account security measure. Starting this spring, all Nest accounts will require two-factor authentication.
Nest has long offered two-factor support, but only via SMS. The Google account migration first offered last year added some additional account security, though. Those who haven't activated two-factor codes or migrated to a Google account will be affected by the new policy. When the change goes into effect, those users will begin getting two-factor codes for each login via email. If you don't input the code, you can't log in. The Google blog post includes several other notable security features like login alerts and password checks, but none of those are new.
Forcing 2FA means an attacker would need to have access to both your Nest account and your email to get in. That is, of course, much less likely. Google's language doesn't make it clear whether or not you'll be able to use a Google account login sans 2FA to access Nest. We've reached out to Google and will update when we hear back.
Google account clarification
Google has given us a little more background on how this will affect those who already migrated to a Google account. The new 2FA codes will only be required for those with unprotected Nest accounts. Those using Google logins won't have to deal with additional one-time codes. This is true even if your Google account is not protected by 2FA (it really should be, though).