Coin
When I first saw the video for Coin, I thought it was pretty interesting. The major concern is security. This would allow anyone in possession of any card to add it to their Coin (although the FAQs says you can't. I don't see what would be stopping a user from putting someone else's name).
Another question is whether retail stores will accept Coin since it is not the original credit card. I can definitely see some places refusing such payment since it is a change and not common.