I really hope they don't complicate an otherwise zen-like experience.
And this wouldn't fix the issue at all... I bet that 80%+ of users would leave the default post@ submission address.
That seems like a very dogmatic attitude. Security almost always comes at some cost (e.g., inconvenience), and sometimes that cost is not worth the benefit.
And I stand by what I wrote earlier, if you want to add some security options then they should be enabled by default. Having even the best security system in-place is useless when it's disabled. Isn't it?
I use a service called Postful to send snail mail via email, that has security along these lines -- I email them a PDF with a mailing address in the subject line, and they post a letter and charge me a buck. They give security options (in my case, I include a "secret" word in the subject line, and there's a confirmation link that's emailed to me), but if I wanted, I could even let anyway send email to a given postful.com address, and it would mail a letter on my dime with no confirmation.
I haven't heard about any abuses.
This isn't UUCP, the message will go from A to B across the internet backbone. There will only be SMTP relays along the way if either your email host or the receiver's email host has chosen to set things up that way. We'd have a much bigger problem with internet security if everyone's email was relayed through questionable servers as a matter of course.
More secure than no password, but not secure.
More to the point, what you are saying is that your ISP can read your unencrypted internet traffic. This is not news.
This way, grandpa talking about his dog doesn't need to bother learning about security he doesn't really care about and the power user can post securely if it so happens that someone decides to spam his blog
If your address is gmail and my address is gmail, our mx domain has the same spf record and same IPs. Sure, some mail servers will prevent you from authenticating with one ID and sending as another, but many others will let that slide.
Gmail won't. Hotmail won't. Yahoo won't. In fact I can't think of a single authenticating mail service that doesn't also validate authorization (e.g. you can only send as you).
Now maybe you're going to point out micro-mail servers, but that's kind of beside the point because very few other people share it with you. e.g. you used gmail as your example, but services like gmail aren't vulnerable to that.
Certainly not ideal. Typos would confuse matters and the idea of secret word authentication is not exactly common/obvious for the masses.