Read Dalton's response http://daltoncaldwell.com/a-response-to-brennan-novak the only thing missing is the free user invites and open source codebase, however someone has already made an iOS app to the platform https://github.com/sneakyness/AppApp
I've always felt that not open sourcing goes against most of Dalton's arguments with App.net. I also realize open sourcing may kill the project under it's own weight. It's a tough call but I really wish they would do it :).