Earlier quoted context omitted.
I listen to the podcasts of precisely six people . Four of those six people have already voiced support for join.app.net ((Dan Benjamin, John Grueber, Marco Armenti, John Siracusa). I expect the other two would join fairly quickly, and, given the "open" nature of join.app.net, I expect anybody who wanted cross-posting from twitter over to join.app.net, wouldn't have much difficulty. Just have the client post on both…
Honest question: what stops you from simply browsing a twitter list of those twelve people? Or, more pragmatically: what incentive do I, a person who doesn't really care about 'openness' and uses Twitter for a billion other reasons anyway, have to pay $50 and subscribe to app.net instead of just following the twitter list of those twelve people?
Today you can follow those 12 people, but if this takes off then it promises a return to the nostalgic '06 and '07 of Twitter when the community was small and everyone there was "interesting". The cognoscenti of tech may well abandon Twitter in favor of this un-walled, membership-only garden.
Now who really should care about this? Certainly not most people. It won't compete with Twitter, but it won't have to because it will have a fraction of the infrastructure costs, and it will avoid the eternal september that plagues every popular free service. This will be a niche product, but if successful it will potentially be highly valuable to that niche, and also it will be quite novel as a paid product of this type has never taken off as far as I know. If you really buy into the hype, you might even think that the success of app.net could signal the high water mark of free services as it becomes apparent just how much better a service can be created when it's paid for directly by users.
I'm not saying I believe any of this, but I do see the potential.