Surely google can correlate the contents of your mail with blogs on getting fired/splitting up with your partner/falling out with your relatives and advise you whether or not you should send it.
There was some scripting built in, and one of the things you could do was add a one hour sending delay to anything with a certain number of chilis.
The combination of visual display and scripted "let me sleep on it" features worked really well.
Similarly, email clients should catch references to attachments that aren't there, and ask "do you really want to send this without an attachment?"
Even more generally, they could recognize (and allow to be configured) certain addresses as requiring additional confirmation before sending -- so a mistaken overbroad or misdirected CC becomes less common.
(I'm thinking: first time you send to an address, you get a confirmation popup, and a chance to classify this address: require dialog confirm; require captcha/retype-of-name confirm; require waiting period; require confirm if combined with other address; etc.)
This was a Better GMail (Firefox extension) feature that is now available in GMail Labs under Settings.