What's hard about GPG or S/MIME-encrypted mail? You set up the thing to integrate with the MUA once (in GPG's case, S/MIME are supported out-of-the-box with most common desktop MUAs), then the only hassle is to enter password on startup or when reading the first message. And lock the computer properly when you're getting away.
Write a message, tick "encrypt" (or don't untick it), send, done. Receive a message, see a badge "encrypted, verified", type a password (if key's not cached before), read it as usual, done. I fail to see how anything can be easier and less obstructive than this.
At least, my only problem with encrypting email is that practically no one of my peers have keys published. This could be easily solved if mail client software vendors could make their products ask user to generate and backup a keypair on install.
Oh, right, encryption has problems with webmail. Extension/userscript kludges are insecure (unless they open separate window/tab for anything private) and break with every other update.