The problem with Twitter and Facebook is that they've moved away from supporting external developers into media companies (so the argument goes).
If that's true, then Twitter and Facebook are looking for the best interests of it's advertising partners like every media company (radio, television, print, etc.) rather than the consumers.
In the case of Facebook and Twitter, it means rolling out features that benefit the advertisers. Now, it is certainly possible that the features rolled out could benefit both sides, but the moment Twitter/Facebook has to make a decision between the two they'll pick the advertisers since that's how they butter their bread.
App.net, by contrast, only has one option which is make the best choices for the developers/users. In some sense, this is really more of a B-to-B venture than a B-to-C since, ideally, App.net focuses on the API and let's the developers build the robust applications.
Frankly, I like this idea.