I'm sorry for piling on, but the fact that drafts aren't encrypted is a huge deal. This is a gaping hole, and greatly reduces the usefulness of your extension. When I encrypt mail, I want to know that only me and my recipient can see it. I don't trust Google any more than any other site. Their interest is in making money; not necessarily in protecting my privacy. And since a large part of their business model involves analyzing (reading) people's mail (Gmail), I'm not inclined to trust them very far.
I use Enigmail and Thunderbird currently to send encrypted mail via Gmail. I've even set my girlfriend and some friends up this way. They're not very savvy, technologically, and they prefer the Gmail web interface. Your plugin is almost there! If you encrypted drafts, you really would have hit on the holy grail.
Disabling drafts would be good. If that doesn't work, how feasible would it be to encrypt drafts for yourself before they're auto-saved, and decrypt them when they're opened for editing?
