Peter Bright and Dan Goodin for Ars Technica:
If you don't want a government, service provider, employer, or unauthorized party to have access to your mail at rest, you need to encrypt the mail itself. But most encryption algorithms are symmetric, meaning that the encryption key serves a dual purpose: it both encrypts and decrypts. As such, people encrypting mail with a symmetric key would be able to decrypt other mail that used the same symmetric key. While this would protect against anyone without the key, it wouldn't be very useful as an encrypted e-mail system.
The solution to this is asymmetric cryptography. In asymmetric encryption there are two opposite keys, and a message encrypted with one key can only be decrypted with the other. The two keys are known as a private key, which as the name might suggest is kept private, and a public key, which is broadcast to the world. Each time you want to send an e-mail to someone, you encrypt it with the recipient's public key.
I first learned about PGP (Pretty Good Privacy) in 1994, back when "going online" meant disabling call waiting, using a modem, and dialing a BBS. (Tell us another one about the ancient times before the Internet, grandpa!) I told my friends and family about the importance encryption in 2001 or 2002, soon after the PATRIOT Act passed.
Despite the spike in privacy concerns around that time, no one I knew besides a few geeky friends saw much of a need even for digital signatures, much less wholesale encryption. As a result, encryption was basically useless to me, even though I saw a clear benefit to using it. If the people you are communicating with aren't using encryption, then you can't realize any benefit either.
Metadata will still paint a connection between sender and receiver even if you use encryption. While there are ways around this, they are so tedious and convoluted that only those who really need to circumvent surveillance would use them. No routine communications would be worth the trouble. The observation that security and convenience are inversely related is not a novel one.
To gain widespread use, encryption would have to be pre-installed, automatic, opt-out instead of opt-in, and end-to-end. Right now, There are only two widespread mobile voice/video services I know of that use encryption by default and without explicit user interaction: Skype and FaceTime.
Interestingly, authorities have complained about iMessage text communications being encrypted, something which few normal people even noticed until this story about wholesale information gathering broke.