The way these guys pump out features, I don't see how any other blogging service can keep up. If other services try to add more engineers it will only slow down their process. Goes to show there's no better way to build a product than with a small group of awesome, dedicated coders.
I think posterous is awesome, and it has become my primary blog, although I don't blog much. That said (and no offense intended), this feature is not exactly groundbreaking. Livejournal, Wordpress, and MoveableType all had stuff like this five years ago, right?
My understanding of how posterous works is to select and implement, cleanly, core functionality in a way that keeps the interface simple. I don't think that posterous out-competes other blog websites (or software) on features.
The most important feature in a blogging platform is the content that's already on it, and in that sense, Posterous still has a lot of catching up to do. More content drives organic growth through links and other SEO mechanisms, which in turn drives more users to sign up.
And when and if you do get that huge growth, all these cool features don't always scale well. I think scaling issues is what really slows down innovation with the big guys.
