If Google Contribute (https://www.google.com/contributor/welcome/) doesn't pull a Google and remain invite only until death, it'll simplify the process and we might see this become a more common approach.
I really don't like the idea of every site attempting to roll out their own solution. It would be nice to have a system to manage subscriptions across a variety of websites. I log-in, and I can see all the sites I'm subscribed to at the moment. I can then toggle individual sites on or off with the push of a button, or I can change payment plans for individual sites (if I switch to the HN $2/mo plan, my username appears in blue). Sites could then query the payment info using some kind of API and user tokens, to see what user is currently subscribed to what plan.