I don't agree with the comments here and on TechCrunch about it allowing people who don't understand how to blog to create an account. Those same people are pretty unlikely to even have an email account in the first place.
I know plenty of people who use email regularly but claim ignorance as to what a blog is. In fact, I'm related to a few. My guess is that I could get them up and running with Posterous pretty quickly.
Yeah, of course every person isn't like that. It's still true that the majority of people simply do not care about sharing what they do online, no matter what it's called or how they register for it.