Startup Idea: Shopify for SaaS companies

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.