The problem with App.net (as it's currently described, anyway) is that it's halfway between what people who love Twitter want and what people who hate Twitter want, which makes it less than an obvious sale for both.
People who love Twitter want... Twitter. App.net doesn't interoperate with Twitter; it's another, separate silo. There's no guarantees that your Twitter friends will move to App.net with you, which limits the appeal if you live in Twitter.
People who hate Twitter hate it for a variety of reasons. App.net doesn't address most of these. If you hate Twitter because they kneecapped their developers, guess what, there's no guarantees App.net won't kneecap their developers five years from now either. If you hate Twitter because it's a centralized, proprietary system, guess what, App.net is centralized and proprietary too. If you hate Twitter because of its scaling problems, guess what, there's no guarantees that App.net (which, as noted, is as centralized as Twitter is) will be any better at dealing with that stuff. And so forth.
Personally, I fall into the latter category; I have lots of problems with Twitter. But nothing about App.net screams to me that here is the solution to those problems. Instead it just feels like Twitter with a different owner and a subscription fee -- which are just about the only ways at this point you could take the idea of Twitter and make it less appealing to me.