I was one of the launch investors of App.Net which you may have read about. (Don't get excited - only $50 - I'm no angel). Once it launched, that became the onboarding fee for new joiners and, it's doing so well it has already brought that down to $36.
If you're a developer and want access the the API, that'll set you back £100 per year, and many have already taken the plunge and begun to create their own desktop and mobile apps for the platform.
In the main right now, app.net and the apps which have sprung up around it are more like Twitter than they are different from it. That's not a problem as it's always easier to start from an existing paradigm and already some more adventurous and diverse apps are beginning to emerge. Also, whilst the experience is Twitter-like to a degree, there are some important and fundamental differences. In terms of the actual conversation, each post is restricted to 256 characters. This may not sound a big deal, over and above Twitter's 140 character limit, but it's surprising how liberating it can be and how much more precise you can be in expressing an idea or a comment with that broader palette.
Best of all though, given the paid-for nature of the service, there are no ads or promoted posts. In fact, there are very few brands in there at all just now. Similarly, there aren't too many newsfeeds just yet. In fact it's just real people being, in the main, positive and friendly and having conversations. Even friends who already know each other from Twitter and other platforms are developing their relationships in a way which hasn't been possible elsewhere to date.
Here's CEO Dalton Caldwell explaining the rationale behind the network:
The conversation can get a little technical... naturally there are many app developers talking about developing apps. However, there is so much more going on and given the relatively low numbers of people currently using the service, engagement levels are high and of great quality. Sure it can get a little sniffy about other networks or about people cross-posting from elsewhere, but by and large it's fairly well self-policed.
The membership fee is, without doubt, a barrier to entry for many people. This could well be a good thing. Those people paying to join app.net, whether they are a developer or not, are investing more than their joining fee. They are investing in the people across the network by actively participating in a way that rewards themselves and the other members directly, without generating unwarranted spin-off 'benefits' for third-parties, advertisers etc beyond the network.