I predict that working out not-so-great with shared credit cards, which are an extraordinarily common financial arrangement in many American families. (Perhaps less so among twenty-something techies than the population at large -- I deal with many, many customers who on their husband's card.)

Worth mentioning: not everyone has a credit card. A requirement that you have to own a credit card will, in effect if not in design, essentially redline your service. Reasonable people can disagree about how much that matters, but I would predict that many newsrooms would go nuclear if you connected the dots for them on the inability of, e.g., 60% of blacks in Chicago from being barred from civic participation on $PICK_AN_ISSUE because they don't have a bank account. (There are prepaid cards to circumvent that restriction. Of course, the riffraff who are the intended targets of the policy are also capable of buying prepaid cards.)