So what are some of the reasons you aren't backing?
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.
To me it seems like Twitter pissed this guy off so he decided to try and build a competitor. It's a nice idea (and something us geeks can get behind) but if he did any market research I bet he would have discovered very quickly it's a lost cause. The vast majority of people aren't going to pay for a product they can get for free and people are the most important aspect of a social network. Without people it's doomed to failure.
I would absolutely back a p2p twitter like service and also pay for the "official" client and it's updates.
But just another corporate owned service...naaa