First, HDR is not available on the iPhone 3G or 3GS so those who didn't have a 4 would buy the app (and they do). Second, Posterous email-in a blog post is a feature that is available to any blog platform I'm aware of but what they did differently is to focus on it making blogging as simple as can be. It was sort of a reverse engineering of the whole work-flow. That turned into a community and it is that community that is the product. They found a niche within making a company based on a neglected feature. Cute.