I use Posterous and I love it, I use also an iPhone (3G) and that one I love too. The iPhone app of Posterous is however a bit of a disappointment, it can only do pictures. It disappointments me because this way the app adds no value at all. I can do more with the email app on the iPhone than I can do with the Posterous app.
Things I would like in the next version so I am more likely to use it:
Not autoposting to everywhere
There is no need for me that all my content is going to everywhere. The current app only supports 'all', I would prefer to select per post I do on Posterous to select to which other channels it should be distributed.
Text
I take pictures every once in a while, however I prefer to use either Flickr or mobypicture for that. Every once in a while I want to share a picture on Posterous and that works fine. However I am more a text guy, I like to put some sentences in a certain sequence and call it a blog. The current app doesn't support that, the horrendous mail app of the iPhone does.
Safari bookmarklet
The tool bookmark (an app for delicious) on the iPhone offers the option for creating a bookmark in Safari that enables you to share a site easily with the app. It would be great for Posterous to do that while browsing some sites on a too little window of your iPhone
Subscriptions
I love to know what the people are posting on Posterous, especially of the blogs I subscribed at. Would be great to be able to take a look in the Posterous app to see what the new updates are in my subscriptions
My PosterousAs a proper web2.0 narcissist I would like to know how things are going at my little at Posterous, views, favorites, but also comments (and to option to reply at those comments) would be really great.