App.net, the paid social network
I've always found the value proposition of App.net to be rather clear. In fact, I was one of the really early ones to back App.net because I liked the idea so much. I think it can be compared to a free gym versus an expensive one. The free gym is nice, because it doesn't cost me anything, so I go there. But since it's free there're all kinds of things that bug me: There're way too many people, I always have to wait till I can use a device, the devices are old and of bad quality, and especially since it's free I don't feel very inclined to really use it. I go there a couple of times, and that's it. Now the expensive gym is top class everywhere, the devices are great, the people I meet there are really interesting, and since I pay a lot per month, I'm also interested in using it as much as possible. Loss aversion makes me actually go there quite often in order to not get a bad conscience (since I paid so much for it). It's the same with app.net I think: All the people there actually care. The quality of discussion is really good, people are friendly, there're tons of interesting things going on, the API is really good and we as developers get a say in how it should be changed. They actually ask and listen for advice to make the best API possible. I've worked (a lot) with the Facebook, Twitter, and Instagram API, and in all these examples, there're always things that suck, and you can't do anything about it. On ADN, there's actually a good chance that it'll be fixed.
A good API, on the other hand, means that there'll be huge innovation on the client front. Not now; right now, most things look like Twitter, and there're many clients which are all a tad basic. But give it some time to evolve. Soon, ADN will have quality discussions, quality clients, and lots of innovative features since it is so open to changes (as opposed to Twitter, who at some point became very opposed to changes).
If you ask me right now, I'd say the big value proposition of ADN is the quality of the discussion, if you ask me in 5 months, I guess I'll answer that it's the fantastic client ecosystem.
Note that I'm the author of the ADM Mac client Appetizer http://www.instadesk-app.com/appetizer