I don't seen anything artificial or limiting about any of that.
Our goals for Posterous are just totally different than what Tumblr is going for. We're aiming for Wordpress.com, not a microblog.
You should know, you can create photo sets with tumblr, allowing multiple images. i.e. more than just one.
One audio file per week hasn't been around since ver. 1 of tumblr. I'm not exactly sure where you're drawing your talking points, but they're simply...incorrect.
Posterous has some nice features but its place in the market seems a little undefined. It's more than Tumblr and less than Wordpress.com, which with the widgets available for hosted sites can do pretty much everything Posterous does (if at times in a less attractive way) and quite a bit more.