Most of the things you mention are already available: sign-up from Facebook/Twitter/etc and payments Stripe/Recurly/Paypal.
The fact is that generally those things need to be customized for the specific needs of each product even more if you're not building a generic site, ie. a forum or an intranet. And then offering a sufficiently easy to use API to do rather complex things ends up "bounding" the ability to extend it. (Which is is what happens when people want to customize modules for popular CMSs like Wordpress.)
Also there are also frameworks that take care of this Django/Rails.
Great idea!
I love this type of projects, and mobile is a great niche to work on nowadays.
Some feedback: it's a bit annoying that the "Show more ???" ajax buttons send you everytime to the top of the page.
