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.
The final straw was posting code. Tumblr makes that stupid easy (since they accept Markdown), so I can just do that instead of trying to figure out the magic incantation to make it work on Posterous (gists or something? I don't know. It wasn't documented when I last tried).
Markdown etc has been a much-wanted feature, even for ourselves internally. We try to focus on normals as much as possible, so it hasn't hit the top of our priority list yet, but it has been rising in the list without a doubt.
I will say though that their service is dead simple to use. But then again the Facebook and Flickr exporters are pretty easy to use, too.