App.net is the first major social platform to charge users to be a part of the network. The immediate advantage to this is it provides a barrier to entry for spammers.
Spam has become a huge problem on Twitter, for example, many times when your Tweets contain a keyword like iPad, you will get many spam @replies and direct messages.
While many Twitter users have complained about the price ($50/year), it actually makes for a better user experience. By not providing services for free, App.net users are the business and not the product.
On free social networks (and other web services), the user is the product. This makes the developers more responsive to the needs of their business (advertisers) and less responsive to the needs of their users.
One big example of how being the product is not a good thing is data portability. If a service you are using goes under or you want to switch to a different provider, you would expect to be able to take your data with you. As the product, the service providers have no incentive to allow you to take your data elsewhere. If you take a look on Twitter
and Facebook, you will find no native way to export your data.
While App.net is still in its infancy I hope that the service continues to improve and listen to its users. With the activity of the CEO, Dalton Caldwell in response to user comments, I think they are on the track to success.